Try setting different header for FreeBSD.

spec/sys_proctable_bsd_spec.rb

@@ -221,6 +221,10 @@
   context 'C struct verification' do
     let(:dummy){ Class.new{ extend Mkmf::Lite } }
 
+    before do
+      @header = RbConfig::CONFIG['host_os'] =~ /dragonfly/i ? 'sys/kinfo.h' : 'kvm.h'
+    end
+
     it 'has a timeval struct of the expected size' do
       expect(Sys::ProcTableStructs::Timeval.size).to eq(dummy.check_sizeof('struct timeval', 'sys/time.h'))
     end
@@ -242,7 +246,7 @@
     end
 
     it 'has an kinfo_proc struct of the expected size' do
-      expect(Sys::ProcTableStructs::KInfoProc.size).to eq(dummy.check_sizeof('struct kinfo_proc', 'sys/kinfo.h'))
+      expect(Sys::ProcTableStructs::KInfoProc.size).to eq(dummy.check_sizeof('struct kinfo_proc', @header))
     end
   end
 end