Mark all curve25519 tables const.

See also upstream's dc22d6b37e8058a4334e6f98932c2623cd3d8d0d. (Though I'm not
sure why they didn't need to fix cmov.)

Change-Id: I2a194a8aea1734d4c1e7f6a0536a636379381627
Reviewed-on: https://boringssl-review.googlesource.com/7280
Reviewed-by: Adam Langley <agl@google.com>

crypto/curve25519/curve25519.c

diff --git a/crypto/curve25519/curve25519.c b/crypto/curve25519/curve25519.c
index 272dfcf..737b7b9 100644
--- a/crypto/curve25519/curve25519.c
+++ b/crypto/curve25519/curve25519.c
@@ -1204,7 +1204,7 @@
   return y;
 }
 
-static void cmov(ge_precomp *t, ge_precomp *u, uint8_t b) {
+static void cmov(ge_precomp *t, const ge_precomp *u, uint8_t b) {
   fe_cmov(t->yplusx, u->yplusx, b);
   fe_cmov(t->yminusx, u->yminusx, b);
   fe_cmov(t->xy2d, u->xy2d, b);
@@ -1362,7 +1362,7 @@
 #else
 
 /* k25519Precomp[i][j] = (j+1)*256^i*B */
-static ge_precomp k25519Precomp[32][8] = {
+static const ge_precomp k25519Precomp[32][8] = {
     {
         {
             {25967493, -14356035, 29566456, 3660896, -12694345, 4014787,
@@ -3653,7 +3653,7 @@
   }
 }
 
-static ge_precomp Bi[8] = {
+static const ge_precomp Bi[8] = {
     {
         {25967493, -14356035, 29566456, 3660896, -12694345, 4014787, 27544626,
          -11754271, -6079156, 2047605},