Const-correct bn_gather5.

Not that we get much type-checking from this, as an assembly function.

Change-Id: I21643444cfc577e2d68f11891e602724ded52e7f
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52831
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/bn/internal.h

diff --git a/crypto/fipsmodule/bn/internal.h b/crypto/fipsmodule/bn/internal.h
index 79b25b04..50fd362 100644
--- a/crypto/fipsmodule/bn/internal.h
+++ b/crypto/fipsmodule/bn/internal.h
@@ -391,7 +391,7 @@
 
 // bn_gather5 loads index |power| of |table| and stores it in |out|. |out| and
 // each entry of |table| are |num| words long. |power| must be less than 32.
-void bn_gather5(BN_ULONG *out, size_t num, BN_ULONG *table, size_t power);
+void bn_gather5(BN_ULONG *out, size_t num, const BN_ULONG *table, size_t power);
 
 // bn_power5 squares |ap| five times and multiplies it by the value stored at
 // index |power| of |table|, modulo |np|. It stores the result in |rp|. The