In case a toolchain supports OpenMP (e.g: external toolchains), just
disable support for it in libsoxr since cmake would detect that and
later we would fail to create the package because there is no libgomp
package being created.

In case OpenMP is desireable and this is supported by the toolchain, we
should introduce a specific option for that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>