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/api/buffer.c

@@ -1,6 +1,7 @@
 // Much of this code was adapted from 'if_py_both.h' from the original
 // vim source
 #include <errno.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>