tcg/mips: delete commented out extern keyword.

Delete commented out extern keyword on link_error().
Signed-off-by: NJiang Biao <jiang.biao2@zte.com.cn>
Message-Id: <1506762042-32145-1-git-send-email-jiang.biao2@zte.com.cn>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

tcg/mips/tcg-target.inc.c

@@ -36,7 +36,7 @@
 #else
 /* To assert at compile-time that these values are never used
    for TCG_TARGET_REG_BITS == 64.  */
-/* extern */ int link_error(void);
+int link_error(void);
 # define LO_OFF  link_error()
 # define HI_OFF  link_error()
 #endif