Include stdbool.h in some files which use it

Done by manual inspection of the output of this script:
grep -r -l -w "bool\|true\|false" * | grep 'c$\|h$' > has_bool
grep -r -l "stdbool.h" * | grep 'c$\|h$' > has_include
grep -F -x -v -f has_include has_bool

src/nvim/syntax.c

@@ -13,6 +13,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <inttypes.h>
+#include <stdbool.h>
 #include <string.h>
 #include <stdlib.h>