egl: bring back the default glapi.so name

Earlier commit replaced the default platform specific libglapi.so name
with an #error.

This may have been overzealous since the name is the correct for the BSD
platforms, at least. Reinstate the hunk - bringing back OpenBSD, et al.
to a successful build state.

Fixes: 7a9c92d071 ("egl/dri2: non-shared glapi cleanups")
[Emil Velikov: format the patch from Eric, add commit message and tag.]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/egl/drivers/dri2/egl_dri2.c

@@ -2808,10 +2808,8 @@ dri2_load(_EGLDriver *drv)
    const char *libname = "libglapi.0.dylib";
 #elif defined(__CYGWIN__)
    const char *libname = "cygglapi-0.dll";
-#elif defined(__linux__)
-   const char *libname = "libglapi.so.0";
 #else
-#error Unknown glapi provider for this platform
+   const char *libname = "libglapi.so.0";
 #endif
    void *handle;