Texture::releaseTexImage() is a virtual method called by libEGL but
defined by libGLESv2, so prevent LTO from treating it as undefined.

This fixes running Chromium swiftshader_unittests with an 'is_official'
build.

Bug chromium:720933

Change-Id: I58c4441f9bd32b96703a28267837cc79b6087659
Reviewed-on: https://swiftshader-review.googlesource.com/19708Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>