bin: use a full path to build plugins.dat

This needed on Windows as LoadLibraryEx doesn't like relative pathes and thus
building on Windows doesn't go to the end.

The resulting pathes in plugins.dat are still relative and so are the ones
resulting from `make package-win-common` that can be distributed.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

bin/Makefile.am

@@ -116,7 +116,7 @@ endif
 ../modules/plugins.dat: vlc-cache-gen$(EXEEXT)
 	$(AM_V_at)rm -f ../modules/plugins.dat
 	$(AM_V_GEN)if test "$(build)" = "$(host)"; then \
-		./vlc-cache-gen$(EXEEXT) ../modules ; \
+		./vlc-cache-gen$(EXEEXT) `realpath ../modules` ; \
 	else \
 		echo "Cross-compilation: cache generation skipped!" ; \
 	fi