Fix typos in BUILD.gn source lists

"foo,cc" is not the same as "foo.cc"

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/326193002

Patch from James Robinson <jamesr@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21765 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

BUILD.gn

@@ -901,7 +901,7 @@ source_set("v8_base") {
     }
   } else if (is_android) {
     defines += [ "CAN_USE_VFP_INSTRUCTIONS" ]
-    sources += [ "src/platform-posix,cc" ]
+    sources += [ "src/platform-posix.cc" ]
 
     if (build_os == "mac") {
       if (current_toolchain == host_toolchain) {
@@ -916,7 +916,7 @@ source_set("v8_base") {
       }
     }
   } else if (is_mac) {
-    sources += [ "src/platform-macos,cc" ]
+    sources += [ "src/platform-macos.cc" ]
   } else if (is_win) {
     # TODO(jochen): Add support for cygwin.
     sources += [