Fix typo in gn ccache support for linux:x86

BUG=

Review URL: https://codereview.chromium.org/1308403004

Cr-Original-Commit-Position: refs/heads/master@{#345355}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0127ddc1c60d51b1925eb8b2a3740147d5d34de5

toolchain/linux/BUILD.gn

@@ -53,7 +53,7 @@ gcc_toolchain("clang_x86") {
 
 gcc_toolchain("x86") {
   cc = "${compiler_prefix}gcc"
-  cxx = "$compiler_prefix}g++"
+  cxx = "${compiler_prefix}g++"
 
   readelf = "readelf"
   nm = "nm"