ppp: install pkg-config file on InstallDev

It seems some package (sstp-client) makes use of pppd.pc file to detect
the ppp version as 2.5.0 changed some API.

Also install the .pc file to permit the version detection of pppd.

package/network/services/ppp/Makefile

@@ -192,6 +192,8 @@ endif
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/include
 	$(CP) $(PKG_INSTALL_DIR)/usr/include/pppd $(1)/usr/include/
+	$(INSTALL_DIR) $(1)/usr/lib/pkgconfig
+	$(CP) $(PKG_INSTALL_DIR)/usr/lib/pkgconfig/*.pc $(1)/usr/lib/pkgconfig
 endef
 
 define Package/ppp/script_install