Include limits.h for MAX_PATH

ade6d0e included changes in config.c
to use realpath(3) to determine the full path of a config file,
declaring a char array of size PATH_MAX for storage - except this broke
because PATH_MAX is defined in limits.h which wasn't included.

src/config.c

@@ -38,6 +38,7 @@
 
 #include <stdio.h>
 #include <unistd.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <assert.h>
 #include <string.h>