(platform) add missing tempfile.c

This should've been added with the keymap forwarding code.

src/platform/posix/tempfile.c

@@ -0,0 +1,30 @@
+#include <stdlib.h>
+#include <unistd.h>
+
+int arcan_strbuf_tempfile(const char* msg, size_t msg_sz, const char** err)
+{
+	char filename[] = "arcantemp-XXXXXX";
+	int state_fd;
+	if (-1 == (state_fd = mkstemp(filename))){
+		*err = "temp file creation failed";
+		return -1;
+	}
+	unlink(filename);
+
+	size_t ntw = msg_sz;
+	size_t pos = 0;
+	while (ntw){
+		ssize_t nw = write(state_fd, &msg[pos], ntw);
+		if (-1 == nw){
+				continue;
+			*err = "failed to write";
+			close(state_fd);
+			return -1;
+		}
+		ntw -= nw;
+		pos += nw;
+	}
+
+	lseek(state_fd, SEEK_SET, 0);
+	return state_fd;
+}