Add 2 files for multiplication in GF(128)

Multiplication in GF(128) is required by the AES-XEX mode for computing
X in the XEX formula from
https://en.wikipedia.org/wiki/Disk_encryption_theory#Xor-encrypt-xor_.28XEX.29
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 60fa7f8..03d3f6c 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -29,6 +29,7 @@
     entropy_poll.c
     error.c
     gcm.c
+    gf128mul.c
     havege.c
     hkdf.c
     hmac_drbg.c