Use XMEM_NEQ instead of mem_neq

mem_neq is no more used directly. XMEM_NEQ is used instead,
in the same way XMEMCMP, XMEMCPY,... are.

Signed-off-by: Pascal Brand <pascal.brand@st.com>

src/headers/tomcrypt_custom.h

@@ -45,6 +45,9 @@
    #endif
 #define XMEMCMP  memcmp
 #endif
+#ifndef XMEM_NEQ
+#define XMEM_NEQ  mem_neq
+#endif
 #ifndef XSTRCMP
    #ifdef strcmp
    #define LTC_NO_PROTOTYPES