Ensure Integer::GMP_VERSION exists

Do the same in ruby/ruby#10875 for debian
packaging

Gbp-Pq: Name Ensure-Integer-GMP_VERSION-exists.patch

Author: sorah

test/ruby/test_bignum.rb

@@ -634,6 +634,10 @@ def test_even
     assert_equal(true, BIGNUM_MIN.even?)
   end
 
+  def test_gmp_deb
+    assert_kind_of(String, Integer::GMP_VERSION) # https://github.com/ruby/ruby/pull/10875
+  end
+
   def test_interrupt_during_to_s
     if defined?(Integer::GMP_VERSION)
       return # GMP doesn't support interrupt during an operation.