meson.build: tweak sdl-image error message

Before the change missing SDL was reported as:

    ../meson.build:253:4: ERROR: Expected 1 arguments, got 2.

After the error as:

    ../meson.build:258:4: ERROR: Problem encountered: sdl-image required, but SDL was not found

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: "Marc-André Lureau" <marcandre.lureau@redhat.com>
CC: "Philippe Mathieu-Daudé" <philmd@redhat.com>
CC: Rafael Kitover <rkitover@gmail.com>
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200908074016.2593596-1-slyfox@gentoo.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>