plugin: Blind build fix for OSX...

According to a internet search PATH_MAX is in limits.h on OSX.
diff --git a/src/plugin.c b/src/plugin.c
index e953508..3ba2d43 100644
--- a/src/plugin.c
+++ b/src/plugin.c
@@ -29,6 +29,7 @@
 #include <dlfcn.h>
 #include <stdio.h>
 #include <errno.h>
+#include <limits.h>
 
 #include <osmocore/plugin.h>