romcc: Increase buffer size to fit max string size

On the updated builder image, the build is failing because the system
compiler has been updated to GCC 8.2.0.  It complains about the
possibility of overflow when putting one 30 character buffer plus 2
characters into another 30 character buffer.  To fix this, increase
the recipient buffer size by 2.

romcc.c:3645:2: note: 'sprintf' output between 3 and 32 bytes into a destination
of size 30 [-Werror=format-overflow=]
  sprintf(buf, "\"%s\"", scratch);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
romcc.c:3649:2: note: 'sprintf' output between 3 and 32 bytes into a destination
of size 30 [-Werror=format-overflow=]
  sprintf(buf, "\"%s\"", scratch);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: I7879a7202cc3ff52301b10118fc49fcc601f133e
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/c/29850
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/romcc/romcc.c

@@ -3629,7 +3629,7 @@ static void register_builtin_macro(struct compile_state *state,
 
 static void register_builtin_macros(struct compile_state *state)
 {
-	char buf[30];
+	char buf[32];
 	char scratch[30];
 	time_t now;
 	struct tm *tm;