(CI) unbreak FreeBSD build (#286)

* (shmif) add missing header after 8578606

src/shmif/arcan_shmif_server.c:381:4: warning: call to undeclared function 'kill'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  381 |                         kill(*pid, SIGKILL);
      |                         ^
src/shmif/arcan_shmif_server.c:381:15: error: use of undeclared identifier 'SIGKILL'
  381 |                         kill(*pid, SIGKILL);
      |                                    ^
src/shmif/arcan_shmif_server.c:420:4: warning: call to undeclared function 'kill'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  420 |                         kill(cl->pid, SIGKILL);
      |                         ^
src/shmif/arcan_shmif_server.c:420:18: error: use of undeclared identifier 'SIGKILL'
  420 |                         kill(cl->pid, SIGKILL);
      |                                       ^

* CI: skip install step to reduce noise

* CI: disable copyback to avoid spurious failures

See also mpv-player/mpv@e7902eb83dc6

.github/workflows/dragonfly.yml

@@ -10,6 +10,7 @@ jobs:
     - name: Test in DragonFly VM
       uses: vmactions/dragonflybsd-vm@v0
       with:
+        copyback: false
         prepare: |
           pkg install -y cmake ninja pkgconf # build
           pkg install -y evdev-proto freetype2 luajit openal-soft sqlite3 # common
@@ -21,4 +22,3 @@ jobs:
         run: |
           cmake -B _build -G Ninja -S src -DBUILD_PRESET:STRING="everything"
           cmake --build _build
-          cmake --install _build

.github/workflows/freebsd.yml

@@ -10,6 +10,7 @@ jobs:
     - name: Test in FreeBSD VM
       uses: vmactions/freebsd-vm@v0
       with:
+        copyback: false
         prepare: |
           pkg install -y cmake ninja pkgconf # build
           pkg install -y evdev-proto freetype2 luajit openal-soft sqlite3 # common
@@ -21,4 +22,3 @@ jobs:
         run: |
           cmake -B _build -G Ninja -S src -DBUILD_PRESET:STRING="everything"
           cmake --build _build
-          cmake --install _build

src/shmif/arcan_shmif_server.c

@@ -5,6 +5,7 @@
 #include <math.h>
 #include <sys/wait.h>
 #include <pthread.h>
+#include <signal.h>
 
 /*
  * This is needed in order to re-use some of the platform layer functions that