scons: protobuf builder, support source paths

Before this patch, the protobuf builder would search for dependencies
only at the build directory. This works if the importing .proto file
imports paths relative to the build directory, but it results in a build
failure if imports are done relative to the source directory of the
importing file.

This patch adds the source directory of the importing file to the set of
paths searched for dependencies, which solves this issue.

Change-Id: I7debd467485a5087276ac005ac08ab01b32cb02e
Signed-off-by: Adrián Herrera Arcila <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55963
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -240,7 +240,8 @@ def protoc_emitter(target, source, env):
     return [root + '.pb.cc', root + '.pb.h'], source
 
 protoc_action = MakeAction('${PROTOC} --cpp_out ${BUILDDIR} '
-        '--proto_path ${BUILDDIR} ${SOURCE.get_abspath()}',
+        '--proto_path ${BUILDDIR} --proto_path ${SOURCE.dir} '
+        '${SOURCE.get_abspath()}',
         Transform("PROTOC"))
 protobuf_builder = Builder(action=protoc_action, emitter=protoc_emitter,
         src_suffix='.proto')