Fixing error in BUILD.gn file.

Bug: webrtc:9251
Change-Id: I4e336c58b059c074d6b0bdd80d1f29ec5b17c290
TBR=phoglund@webrtc.org
Reviewed-on: https://webrtc-review.googlesource.com/76722
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 52da987..cf4a7a5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -14,8 +14,10 @@
       # When compiling with clang-cl, use -imsvc (similar to -isystem in clang)
       # instead of include_dirs (-I), so we don't need to suppress warnings
       # coming from this package in other projects.
-      "-imsvc",
-      rebase_path(baseclasses_dir, root_build_dir),
+      cflags = [
+        "-imsvc",
+        rebase_path(baseclasses_dir, root_build_dir),
+      ]
     } else {
       include_dirs = [ baseclasses_dir ]
     }