configure: Fix spelling of sdl-image in --help

Fixes: a442fe2f
Cc: qemu-trivial@nongnu.org
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20190517183246.11933-1-armbru@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

configure

@@ -1745,7 +1745,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   gcrypt          libgcrypt cryptography support
   auth-pam        PAM access control
   sdl             SDL UI
-  sdl_image       SDL Image support for icons
+  sdl-image       SDL Image support for icons
   gtk             gtk UI
   vte             vte support for the gtk UI
   curses          curses UI