Fix compiling with `CFLAGS` `-Werror=format-security`
See also https://bugs.gentoo.org/517524

--- a/converter/ppm/ximtoppm.c
+++ b/converter/ppm/ximtoppm.c
@@ -117,9 +117,9 @@
 */
     header->bits_channel = atoi(a_head.bits_per_channel);
     header->alpha_flag = atoi(a_head.alpha_channel);
-    pm_asprintf(&header->author,  a_head.author);
-    pm_asprintf(&header->date,    a_head.date);
-    pm_asprintf(&header->program, a_head.program);
+    pm_asprintf(&header->author,  "%s", a_head.author);
+    pm_asprintf(&header->date,    "%s", a_head.date);
+    pm_asprintf(&header->program, "%s", a_head.program);
     /* Do double checking for bakwards compatibility */
     if (header->npics == 0)
         header->npics = 1;