tools:mksyscall fix compilation warning

mksyscall.c:145:19: warning: initialization discards ‘const’ qualifier
from pointer target type [-Wdiscarded-qualifiers]
  145 |   char *pactual = strchr(arg, '|');
      |                   ^~~~~~
Change to use const char* on pointers

Signed-off-by: Serg Podtynnyi <serg@podtynnyi.com>

tools/mksyscall.c

@@ -142,8 +142,8 @@ static void get_actualparmtype(const char *arg, char *actual)
 
 static void get_fieldname(const char *arg, char *fieldname)
 {
-  char *pactual = strchr(arg, '|');
-  char *pstart;
+  const char *pactual = strchr(arg, '|');
+  const char *pstart;
 
   if (pactual)
     {