contrib: modplug: merge Libs.private into Libs for static linking

(cherry picked from commit 534a9321)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/modplug/rules.mak

@@ -16,6 +16,7 @@ $(TARBALLS)/libmodplug-$(MODPLUG_VERSION).tar.gz:
 libmodplug: libmodplug-$(MODPLUG_VERSION).tar.gz .sum-modplug
 	$(UNPACK)
 	$(UPDATE_AUTOCONFIG)
+	$(call pkg_static,"libmodplug.pc.in")
 	$(MOVE)
 
 .modplug: libmodplug