Add RelWithDebInfo to protobuf build_type list. (#27220)

var/spack/repos/builtin/packages/protobuf/package.py

@@ -60,7 +60,7 @@ class Protobuf(Package):
             description='Enables the build of shared libraries')
     variant('build_type', default='Release',
             description='The build type to build',
-            values=('Debug', 'Release'))
+            values=('Debug', 'Release', 'RelWithDebInfo'))
 
     depends_on('cmake', when='@3.0.2:', type='build')
     depends_on('zlib')