Add missing include to fix build on Mac

Change-Id: Ice97a71e166100cd244371164894169e8fe00f77
diff --git a/src/unix/proc.c b/src/unix/proc.c
index 4c11d0f..746e148 100644
--- a/src/unix/proc.c
+++ b/src/unix/proc.c
@@ -1,6 +1,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <libgen.h>
 #include <limits.h>
 #include <spawn.h>
 #include <stdarg.h>