Use "GMP_DESC" instead of "GPM_DESC"

This seemed to be the only place in the code that was using this
particular transposition.  And, indeed, when compiling with
"GMP_DESC", it looks like it is necessary to disable Diffie-Hellman.
(Otherwise, the test fails for me.)

src/headers/tomcrypt_custom.h

@@ -306,8 +306,8 @@
 /* #define LTC_RSA_BLINDING */
 
 /* Include Diffie-Hellman support */
-#ifndef GPM_DESC
-/* is_prime fails for GPM */
+#ifndef GMP_DESC
+/* is_prime fails for GMP */
 #define MDH
 /* Supported Key Sizes */
 #define DH768