Add `libgmp-dev` as a dependency in README

[ecomaikgolf]

As NTL is getting built with GMP support (thirdparty/build.sh):

./configure SHARED=on NTL_GMP_LIP=on NTL_THREADS=on NTL_THREAD_BOOST=on NTL_EXCEPTIONS=on  \

It's missing when compiling if not installed

g++ -I../include -I.  -g -O2 -pthread -march=native   -o gen_gmp_aux gen_gmp_aux.cpp  -lgmp  -lm
gen_gmp_aux.cpp:29:10: fatal error: gmp.h: No such file or directory
   29 | #include <gmp.h>
      |          ^~~~~~~

Compilation was done with a fresh default Ubuntu 22.04 container