Move -Wno-unused-parameter to general cflags

The GCC build seems to have broke because of this. Turns out
we disable it only on clang, thinking that GCC didn't have that
flag.

Change-Id: Iefeb8e0654ad9f8c6254b3469e3433e1cbd4f88c

build/BUILD.gn

diff --git a/build/BUILD.gn b/build/BUILD.gn
index 667939a..3a48665 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -19,6 +19,7 @@
   cflags = [
     "-Wextra",
     "-Wno-missing-field-initializers",
+    "-Wno-unused-parameter",
   ]
   if (is_clang) {
     cflags += [
@@ -30,7 +31,6 @@
       "-Wno-gnu-zero-variadic-macro-arguments",
       "-Wno-padded",
       "-Wno-reserved-id-macro",
-      "-Wno-unused-parameter",
       "-Wno-weak-vtables",
     ]
   }