Update capstone.h

VS2015: error: C2440: 'initializing': cannot convert from 'const char [4]' to 'char *'
diff --git a/include/capstone/capstone.h b/include/capstone/capstone.h
index 11eb8b4..4899971 100644
--- a/include/capstone/capstone.h
+++ b/include/capstone/capstone.h
@@ -141,7 +141,7 @@
 	// ID of instruction to be customized.
 	unsigned int id;
 	// Customized instruction mnemonic.
-	char *mnemonic;
+	const char *mnemonic;
 } cs_opt_mnem;
 
 // Runtime option for the disassembled engine