GN: Fix error in is_component_build in boringssl

It's trying to append to a list that doesn't exist yet.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/424453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285787 0039d316-1c4b-4281-b951-d872f2087c98

third_party/boringssl/BUILD.gn

@@ -71,7 +71,7 @@ component("boringssl") {
     if (is_posix && !is_mac) {
       # Avoid link failures on Linux x86-64.
       # See http://rt.openssl.org/Ticket/Display.html?id=2466&user=guest&pass=guest
-      ldflags += [ "-Wl,-Bsymbolic" ]
+      ldflags = [ "-Wl,-Bsymbolic" ]
     }
   }
 }