x86: fix build with GCC10

kvm-unit-tests fail to build with GCC10:

/usr/bin/ld: lib/libcflat.a(usermode.o):
  ./kvm-unit-tests/lib/x86/usermode.c:17:  multiple definition of `jmpbuf';
 lib/libcflat.a(fault_test.o):
  ./kvm-unit-tests/lib/x86/fault_test.c:3: first defined here

It seems that 'jmpbuf' doesn't need to be global in either of these files,
make it static in both.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200617152124.402765-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>