Add "libgmp-dev"

[tianon]

See:

• https://github.com/ruby/ruby/commit/83a0709174f9f99038b45914ee54acd7c126f11b
• https://www.npmjs.com/package/gmplib
• https://pypi.org/project/gmpy/ + https://pypi.org/project/gmpy2/
• https://github.com/ncw/gmp