ovmf_git.bb: enable parallel compilation

ovmf_git.bb: enable parallel compilation

The Fedora srpm [1] seems to have no problems with parallel
compilation, so let's also use that for the target. The native
tools however indeed have dependency problems:

| test_Ecc_CParser (CheckPythonSyntax.Tests) ... gcc -o ../bin/EfiRom -L/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/usr/lib -L/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/lib -Wl,-rpath-link,/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/usr/lib -Wl,-rpath-link,/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/lib -Wl,-rpath,/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/usr/lib -Wl,-rpath,/fast/build/ostro/x86/tmp-glibc/sysroots/x86_64-linux/lib -Wl,-O1 EfiRom.o -L../libs -lCommon
| /usr/bin/ld: cannot find -lCommon
| collect2: error: ld returned 1 exit status

ERROR: Task (virtual:native:.../meta/recipes-core/ovmf/ovmf_git.bb:do_compile) failed with exit code '1'

[1] https://src.fedoraproject.org/cgit/rpms/edk2.git/tree/edk2.spec

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>