fix build failure due to missing <stdio.h> (#337)

the following combination currently fails to build:

	$ make HAVE_LIBFONTS=0 HAVE_LIBWEBP=1

this is probably because one of the font library was including <stdio.h>

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/337
Reviewed-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>
Reviewed-by: TAAPArthur <taaparthur@noreply.codeberg.org>
Co-authored-by: NRK <nrk@disroot.org>
Co-committed-by: NRK <nrk@disroot.org>

image.c

@@ -38,6 +38,7 @@ enum { DEF_GIF_DELAY = 75 };
 #endif
 
 #if HAVE_LIBWEBP
+#include <stdio.h>
 #include <webp/decode.h>
 #include <webp/demux.h>
 enum { DEF_WEBP_DELAY = 75 };