loader: include limits.h for PATH_MAX

This is needed to build on illumos.

The location of the PATH_MAX definition in limits.h seems to be fairly standard:
https://pubs.opengroup.org/onlinepubs/009695399/basedefs/limits.h.html

Reviewed-by: Eric Engestrom <eric@engestrom.ch>

src/loader/loader.c

@@ -36,6 +36,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <limits.h>
 #include <sys/param.h>
 #ifdef MAJOR_IN_MKDEV
 #include <sys/mkdev.h>