Fix HRSS build error on ARM

Seeing the following errors with GCC 6 on ARM:

  crypto/hrss/hrss.c:212:12: error: function declaration isn't a prototype [-Werror=strict-prototypes]
   static int vec_capable() { return CRYPTO_is_NEON_capable(); }
              ^~~~~~~~~~~
  crypto/hrss/hrss.c: In function 'vec_capable':
  crypto/hrss/hrss.c:212:12: error: old-style function definition [-Werror=old-style-definition]

Change-Id: Ice540e6d436b8ada1dbc494f1feca10efff11687
Reviewed-on: https://boringssl-review.googlesource.com/c/33624
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
diff --git a/crypto/hrss/hrss.c b/crypto/hrss/hrss.c
index 881da78..dd3f979 100644
--- a/crypto/hrss/hrss.c
+++ b/crypto/hrss/hrss.c
@@ -209,7 +209,7 @@
 // These functions perform the same actions as the SSE2 function of the same
 // name, above.
 
-static int vec_capable() { return CRYPTO_is_NEON_capable(); }
+static int vec_capable(void) { return CRYPTO_is_NEON_capable(); }
 
 static inline vec_t vec_add(vec_t a, vec_t b) { return a + b; }