egl: silence warning

Without this, I get this warning:

../../../src/mesa/src/egl/main/eglapi.c: In function ‘eglDupNativeFenceFDANDROID’:
../../../src/mesa/src/egl/main/eglapi.c:2268:11: warning: ‘ret’ may be used uninitialized [-Wmaybe-uninitialized]
 2268 |    EGLint ret;

Let's initialize ret to EGL_FALSE, like some other functions did when the egl_relax mechanism was introduced.

Fixes: bbef3cb9 ("egl: Relax locking")