Add Netpbm, TGA and XPM MIME types to sxiv.desktop

These formats are also supported by imlib2.

sxiv.desktop

@@ -3,6 +3,6 @@ Type=Application
 Name=sxiv
 GenericName=Image Viewer
 Exec=sxiv %F
-MimeType=image/bmp;image/gif;image/jpeg;image/jpg;image/png;image/tiff;image/x-bmp;
+MimeType=image/bmp;image/gif;image/jpeg;image/jpg;image/png;image/tiff;image/x-bmp;image/x-portable-anymap;image/x-portable-bitmap;image/x-portable-graymap;image/x-tga;image/x-xpixmap;
 NoDisplay=true
 Icon=sxiv