x11_egl: use correct cmake variable name for GLES1 include path

x11_egl: use correct cmake variable name for GLES1 include path

This commit reverts most of 7b4a66f99 that included a hack to search
for include directory next to the library path. This was invalid since
used variable name PKG_GLES1_INCLUDE_DIRS is false, correct name is
GLES1_INCLUDE_DIRS which is properly set by the PKG_CHECK_MODULES call.

This change has been succesfully tested on Fedora and Ubuntu distros by
setting up a custom PKG_CONFIG_PATH.

VK-GL-CTS issue: 1573

Change-Id: I158d816d28f5bbf30288f6226bd5c9e30cf85285
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>