Fix errors caused by -Wformat -Werror=format-security
https://bugs.gentoo.org/show_bug.cgi?id=542134

* xvpbm.c:888:4: error: format not a string literal and no format arguments [-Werror=format-security]
*     if (*pix) fprintf(fp,str1);

--- xv-3.10a/xvpbm.c
+++ xv-3.10a/xvpbm.c
@@ -885,8 +885,8 @@
 	  }
 	}
 	else {
-	  if (*pix) fprintf(fp,str1);
-	       else fprintf(fp,str0);
+	  if (*pix) fprintf(fp,"%s",str1);
+	       else fprintf(fp,"%s",str0);
 	  len+=2;
 	  if (len>68) { fprintf(fp,"\n"); len=0; }
 	}