dri: fix macro name check to detect svga driver

svga driver is detected via HAVE_SVGA.

Since commit 50fc7cc290, svga driver was not loading at all

Fixes: 50fc7cc290 ("glx: directly link to gallium")

Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30307>

src/gallium/targets/dri/dri_target.c

@@ -200,7 +200,7 @@ dri_loader_get_extensions(const char *driver_name)
       return __driDriverGetExtensions_radeonsi();
 #endif
 
-#if defined(HAVE_VMWGFX)
+#if defined(HAVE_SVGA)
    if (!strcmp(driver_name, "vmwgfx"))
       return __driDriverGetExtensions_vmwgfx();
 #endif