fix missing ctype.h include

This file uses isspace but doesn't include ctype.h leading to:
isomac.c: In function 'get_null_defines':
isomac.c:305:30: warning: implicit declaration of function 'isspace' [-Wimplicit-function-declaration]
	for (end = start + 1; !isspace (*end) && *end != '\0'; ++end)