target-mips/translate.c: Add ifdef guard around check_mips64()

The function check_mips64() is only used if TARGET_MIPS64 is defined;
add an ifdef guard to its definition to avoid warnings about it being
unused in other configurations.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
diff --git a/target-mips/translate.c b/target-mips/translate.c
index 8b62e66..c23cb94 100644
--- a/target-mips/translate.c
+++ b/target-mips/translate.c
@@ -1548,6 +1548,7 @@
     }
 }
 
+#ifdef TARGET_MIPS64
 /* This code generates a "reserved instruction" exception if 64-bit
    instructions are not enabled. */
 static inline void check_mips_64(DisasContext *ctx)
@@ -1555,6 +1556,7 @@
     if (unlikely(!(ctx->hflags & MIPS_HFLAG_64)))
         generate_exception(ctx, EXCP_RI);
 }
+#endif
 
 /* Define small wrappers for gen_load_fpr* so that we have a uniform
    calling interface for 32 and 64-bit FPRs.  No sense in changing