configure.ac: add radeonsi to --with-gallium-drivers help string

the help string is used by ./configure --help

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
diff --git a/configure.ac b/configure.ac
index c30bcf0..0052918 100644
--- a/configure.ac
+++ b/configure.ac
@@ -644,7 +644,7 @@
 AC_ARG_WITH([gallium-drivers],
     [AS_HELP_STRING([--with-gallium-drivers@<:@=DIRS...@:>@],
         [comma delimited Gallium drivers list, e.g.
-        "i915,nouveau,r300,r600,svga,swrast"
+        "i915,nouveau,r300,r600,radeonsi,svga,swrast"
         @<:@default=r300,r600,swrast@:>@])],
     [with_gallium_drivers="$withval"],
     [with_gallium_drivers="$GALLIUM_DRIVERS_DEFAULT"])