Make cef_config.h generation a dependency of libcef_dll_wrapper (see issue #2926)

The libcef_dll_wrapper target may be built very early due to minimal
dependencies. Make sure that cef_config.h is generated first as it will be
included via `include/internal/cef_types_linux.h` on Linux.

BUILD.gn

@@ -1291,6 +1291,8 @@ static_library("libcef_dll_wrapper") {
 
   configs += [ ":libcef_dll_wrapper_config" ]
   public_configs = [ ":libcef_dll_wrapper_config" ]
+
+  deps = [ ":cef_make_headers" ]
 }