Add ECDH_compute_key_fips inside the module.

This change adds a function so that an ECDH and the hashing of the
resulting 'x' coordinate can occur inside the FIPS boundary.

Change-Id: If93c20a70dc9dcbca49056f10915d3ce064f641f
Reviewed-on: https://boringssl-review.googlesource.com/30104
Reviewed-by: Adam Langley <agl@google.com>

sources.cmake

diff --git a/sources.cmake b/sources.cmake
index 27fc1c4..f0c953b 100644
--- a/sources.cmake
+++ b/sources.cmake
@@ -40,7 +40,7 @@
   crypto/cmac/cavp_aes128_cmac_tests.txt
   crypto/cmac/cavp_aes192_cmac_tests.txt
   crypto/cmac/cavp_aes256_cmac_tests.txt
-  crypto/ecdh/ecdh_tests.txt
+  crypto/ecdh_extra/ecdh_tests.txt
   crypto/evp/evp_tests.txt
   crypto/evp/scrypt_tests.txt
   crypto/fipsmodule/aes/aes_tests.txt