Restore max() macro for Heimdal

Commit de46edef69 (Remove lowercase min/max macro definitions)
inadvertently removed a definition of max() that is required by
_krb5_n_fold() (src/external/heimdal/krb5/n-fold.c) if no system
definition of max() is available.  For example, on Solaris this results
in a kernel module with an undefined external symbol 'max', which
prevents the kernel module from loading:

  genunix: [ID 819705 kern.notice] /kernel/drv/amd64/afs: undefined symbol
  genunix: [ID 826211 kern.notice]  'max'
  genunix: [ID 472681 kern.notice] WARNING: mod_load: cannot load module 'afs'

Restore the required max() macro definition.

Change-Id: I6bca2bb2b90d7cdbe90a3b769997cdc153f59f6c
Reviewed-on: https://gerrit.openafs.org/15874
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>

src/crypto/rfc3961/krb5_locl.h

@@ -66,6 +66,10 @@
 
 #define ALLOC(X, N) (X) = calloc((N), sizeof(*(X)))
 
+#ifndef max
+#define max(a,b) (((a)>(b))?(a):(b))
+#endif
+
 #ifndef O_BINARY
 #define O_BINARY 0
 #endif