configure: define "pkg-config" in addition to "pkgconfig"

configure: define "pkg-config" in addition to "pkgconfig"

Meson used to allow both "pkgconfig" and "pkg-config" entries in machine
files; the former was used for dependency lookup and the latter
was used as return value for "find_program('pkg-config')", which is a less
common use-case and one that QEMU does not need.

This inconsistency is going to be fixed by Meson 1.3, which will deprecate
"pkgconfig" in favor of "pkg-config" (the less common one, but it makes
sense because it matches the name of the binary). For backward
compatibility it is still allowed to define both, so do that in the
configure-generated machine file.

Related: https://github.com/mesonbuild/meson/pull/12385
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>