proc: pull in ctype.h where needed

Fix the build warnings:

sig.c:227:5: warning: implicit declaration of function 'toupper' [-Wimplicit-function-declaration]
sig.c:231:3: warning: implicit declaration of function 'isdigit' [-Wimplicit-function-declaration]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

proc/sig.c

@@ -8,6 +8,7 @@
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  * GNU Library General Public License for more details.
  */
+#include <ctype.h>
 #include <signal.h>
 #include <string.h>
 #include <stdio.h>