webkitgtk_2.18.6.bb: Fix configure failure for aarch64 build

webkitgtk_2.18.6.bb: Fix configure failure for aarch64 build

Fix configure failure for aarch64

| -- Performing Test CXX_COMPILER_SUPPORTS_-fcolor-diagnostics - Failed
| -- Performing Test CXX_COMPILER_SUPPORTS_-fdiagnostics-color=always
| -- Performing Test CXX_COMPILER_SUPPORTS_-fdiagnostics-color=always -Success
| CMake Error at Source/cmake/OptionsCommon.cmake:42 (WEBKIT_PREPEND_GLOBAL_COMPILER_FLAG):
|   Unknown CMake command "WEBKIT_PREPEND_GLOBAL_COMPILER_FLAG".
| Call Stack (most recent call first):
|   Source/cmake/WebKitCommon.cmake:58 (include)
|   CMakeLists.txt:166 (include)

Signed-off-by: Manjukumar Matha <manjukumar.harthikote-matha@xilinx.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>