Android: enable parallelism for main target too

This adds to the changes that commit

98d16e0d "Android: Tell make about sub-makes to speed up build"

did, and enables parallel builds for minetest
itself as well.

build/android/Makefile

@@ -752,7 +752,7 @@ clean_assets :
 apk: local.properties assets $(ICONV_LIB) $(IRRLICHT_LIB) $(CURL_LIB) $(GMP_LIB) $(LEVELDB_TARGET)       \
 	$(OPENAL_LIB) $(OGG_LIB) prep_srcdir $(ANDR_ROOT)/jni/src/android_version.h    \
 	$(ANDR_ROOT)/jni/src/android_version_githash.h sqlite3_download
-	@${ANDROID_NDK}/ndk-build NDK_MODULE_PATH=${NDK_MODULE_PATH}               \
+	+ @${ANDROID_NDK}/ndk-build NDK_MODULE_PATH=${NDK_MODULE_PATH}             \
 		GPROF=${GPROF} APP_ABI=${TARGET_ABI} HAVE_LEVELDB=${HAVE_LEVELDB}      \
 		APP_PLATFORM=${APP_PLATFORM}                                           \
 		TARGET_LIBDIR=${TARGET_LIBDIR}                                         \