Makefile: Add the leading / to LIBDIR

This change allows the use of an absolute path for LIBDIR. This should
make it easier to locally test changes that use libminijailpreload.so

Bug: None
Test: LIBDIR=${PWD} make && ./minijail0
Change-Id: I8558fce40e916536a52b064a168f4326e4ba8793
diff --git a/Makefile b/Makefile
index e303b50..2a4a28d 100644
--- a/Makefile
+++ b/Makefile
@@ -5,9 +5,9 @@
 BASE_VER=0
 include common.mk
 
-LIBDIR ?= lib
+LIBDIR ?= /lib
 PRELOADNAME = libminijailpreload.so
-PRELOADPATH = \"/$(LIBDIR)/$(PRELOADNAME)\"
+PRELOADPATH = \"$(LIBDIR)/$(PRELOADNAME)\"
 CPPFLAGS += -DPRELOADPATH="$(PRELOADPATH)"
 
 ifneq ($(HAVE_SECUREBITS_H),no)