char is unsigned by default on ppc64

--- a/converter/other/pnmtopalm/palmtopnm.c
+++ b/converter/other/pnmtopalm/palmtopnm.c
@@ -823,7 +823,7 @@
     unsigned int j;
 
     for (j = 0;  j < bytesPerRow; ) {
-        char incount;
+        signed char incount;
         pm_readchar(ifP, &incount);
         if (incount < 0) {
             /* How do we handle incount == -128 ? */
@@ -865,7 +865,7 @@
     unsigned int j;
 
     for (j = 0;  j < bytesPerRow; ) {
-        char incount;
+        signed char incount;
         pm_readchar(ifP, &incount);
         if (incount < 0) {
             /* How do we handle incount == -128 ? */