maint: fix -Werror=suggest-attribute=malloc in expr.c

maint: fix -Werror=suggest-attribute=malloc in expr.c

Add attribute 'malloc' to mpz_get_str to prevent
the following on GCC 8.1.1

  src/expr.c:117:1: error: function might be candidate for attribute
    'malloc' if it is known to return normally
      [-Werror=suggest-attribute=malloc]
   mpz_get_str (char const *str, int base, mpz_t z)
   ^~~~~~~~~~~
  cc1: all warnings being treated as errors

* src/expr.c (mpz_get_str): Add _GL_ATTRIBUTE_MALLOC.