gitignore: Don't ignore proposal_keywords.c

If somebody copies our .gitignore and tries to import the source code,
the proposal_keywords.c file will not be added as it's ignored by the
`*keywords.c` pattern we use to ignore gperf-generated source files.

Closes #2014

.gitignore

@@ -27,6 +27,7 @@ libtool
 y.tab.[ch]
 lex.yy.c
 *keywords.c
+!proposal_keywords.c
 plugin_constructors.c
 Doxyfile
 apidoc/