staging: fsl-mc: Add missing header

Compiling the fsl-mc bus driver will yield a couple of static analysis
errors:
warning: symbol 'fsl_mc_msi_domain_alloc_irqs' was not declared
warning: symbol 'fsl_mc_msi_domain_free_irqs' was not declared.
warning: symbol 'its_fsl_mc_msi_init' was not declared.
warning: symbol 'its_fsl_mc_msi_cleanup' was not declared.

Since these are properly declared, but the header is not included, add
it in the source files. This way the symbol is properly exported.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>