doc: crypto/secp256k1 GMP dependency package name.

Linux build documentation is mostly geared towards Ubuntu 14.04 (LTS).
Appropriate package is called `libgmp-dev` there.

Note that on pristine installations building `geth` with godep will
fail because this header is missing. This is not documented in the
top-level README, but is on the wiki:

https://github.com/ethereum/go-ethereum/wiki/Installation-Instructions-for-Ubuntu#building-geth-command-line-client

That page recommends `libgmp3-dev`, which ATM provides same version
as `libgmp-dev`.

crypto/secp256k1/README.md

@@ -7,8 +7,11 @@ Implements cryptographic operations for the secp256k1 ECDSA curve used by Bitcoi
 
 Installing
 ===
+
+GMP library headers are required to build. On Debian-based systems, the package is called `libgmp-dev`.
+
 ```
-sudo apt-get install gmp-dev
+sudo apt-get install libgmp-dev
 ```
 
 Now compiles with cgo!