kbuild: add a missing line for help message

The help message line for building the source RPM package was missing.
Added it.
Signed-off-by: Jun ASAKA <JunASAKA@zzy040330.moe>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.package

@@ -158,6 +158,7 @@ $(perf-tar-pkgs):
 PHONY += help
 help:
 	@echo '  rpm-pkg             - Build both source and binary RPM kernel packages'
+	@echo '  srcrpm-pkg          - Build only the source kernel RPM package'
 	@echo '  binrpm-pkg          - Build only the binary kernel RPM package'
 	@echo '  deb-pkg             - Build both source and binary deb kernel packages'
 	@echo '  bindeb-pkg          - Build only the binary kernel deb package'