build: fix srcs attribute

Change-Id: Ia3d11644e29500ae5e9b4f6308022b84c411b066
diff --git a/BUILD b/BUILD
index c3f3a68..e938287 100644
--- a/BUILD
+++ b/BUILD
@@ -30,7 +30,7 @@
     outs = [
         "src/trace_processor/metrics/sql_metrics.h",
     ],
-    cmd = "$(location gen_merged_sql_metrics_py) --cpp_out=$@ $SRCS",
+    cmd = "$(location gen_merged_sql_metrics_py) --cpp_out=$@ $(SRCS)",
     tools = [
         "gen_merged_sql_metrics_py",
     ],
diff --git a/tools/gen_build b/tools/gen_build
index 79646b7..d03f8d2 100755
--- a/tools/gen_build
+++ b/tools/gen_build
@@ -504,7 +504,7 @@
       Label(src[src.index('gen/') + len('gen/'):])
       for src in target_desc.get('outputs', [])
     )
-    target.cmd = '$(location gen_merged_sql_metrics_py) --cpp_out=$@ $SRCS'
+    target.cmd = '$(location gen_merged_sql_metrics_py) --cpp_out=$@ $(SRCS)'
     target.tools.update([
       'gen_merged_sql_metrics_py',
     ])