vpx.pc: add missing library for static links

Change-Id: Ic4be3145e8a90d66aa4499416094bd08a9e09af4
diff --git a/libs.mk b/libs.mk
index d4a164d..b63a8f6 100644
--- a/libs.mk
+++ b/libs.mk
@@ -243,6 +243,7 @@
 	$(qexec)echo 'Requires:' >> $@
 	$(qexec)echo 'Conflicts:' >> $@
 	$(qexec)echo 'Libs: -L$${libdir} -lvpx -lm' >> $@
+	$(qexec)echo 'Libs.private: -lpthread' >> $@
 	$(qexec)echo 'Cflags: -I$${includedir}' >> $@
 INSTALL-LIBS-yes += $(LIBSUBDIR)/pkgconfig/vpx.pc
 INSTALL_MAPS += $(LIBSUBDIR)/pkgconfig/%.pc %.pc