st/egl: use *_FILE over *_SOURCES names for filelists

st/egl: use *_FILE over *_SOURCES names for filelists

Silence automake warnings about missing program/library whenever
the _SOURCES suffix is used for temporary variable names.

  warning: variable 'gdi_SOURCES' is defined but no program or
  library has 'gdi' as canonical name (possible typo)

Acked-by: Matt Turner <mattst88@gmail.com>
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reported-by: Johannes Obermayr <johannesobermayr@gmx.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70581
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>