package/libvirt: fix dependencies on kernel headers

package/libvirt: fix dependencies on kernel headers

In ccfc90e1010e (package/libvirt: new package), last-minute changes
missed the depenecny on kernel headers; headers >= 3.12 are required for
all architectures, but AArch64, which requires 4.11 for HWCAP_CPUID:

    ../src/cpu/cpu_arm.c: In function 'virCPUarmCpuDataFromRegs':
    ../src/cpu/cpu_arm.c:562:20: error: 'HWCAP_CPUID' undeclared (first use in this function); did you mean 'HWCAP_PMULL'?
         if (!(hwcaps & HWCAP_CPUID)) {
                        ^~~~~~~~~~~
                        HWCAP_PMULL

Fixes:
 - http://autobuild.buildroot.org/results/85bf7b4dad73a748bf439e63874eb64d9a53088f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - move AArch64 4.11 req. from _ARCH_SUPPORTS to BR2_PACKAGE_LIBVIRT
  - add missing dependency on headers 3.12 for the rest
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>