checkpolicy: always include ctypes.h

The prototype of isdigit() is provided by ctypes.h header. Without
including this file, gcc fails to build checkpolicy using musl libc:

    checkpolicy.c: In function ‘main’:
    checkpolicy.c:705:8: error: implicit declaration of function
    ‘isdigit’ [-Werror=implicit-function-declaration]
        if (isdigit(ans[0])) {
            ^~~~~~~

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
diff --git a/checkpolicy/checkpolicy.c b/checkpolicy/checkpolicy.c
index 2d68316..c82f6f4 100644
--- a/checkpolicy/checkpolicy.c
+++ b/checkpolicy/checkpolicy.c
@@ -57,6 +57,7 @@
  * booleans or conditional rules are thrown away a warning is printed.
  */
 
+#include <ctype.h>
 #include <getopt.h>
 #include <unistd.h>
 #include <stdlib.h>
@@ -73,10 +74,6 @@
 #include <errno.h>
 #include <sys/mman.h>
 
-#ifdef __APPLE__
-#include <ctype.h>
-#endif
-
 #include <sepol/module_to_cil.h>
 #include <sepol/policydb/policydb.h>
 #include <sepol/policydb/services.h>