When measurement-kit is a build dependency (in PKG_BUILD_DEPENDS) of
another package, this package's build dependencies (in DEPENDS) may not
be selected and so may not be built.

(This package does not produce a shared library; the measurement-kit
target package contains a program that is statically linked to the
measurement-kit library. Other packages may include measurement-kit in
their PKG_BUILD_DEPENDS to link to the static library but not include
measurement-kit as a run-time dependency.)

This adds PKG_BUILD_DEPENDS to this package to ensure that its
dependencies are built in this case.

This also adds MEASUREMENT_KIT_BUILD_DEPENDS that dependant packages
should select to ensure that this package's dependencies are correctly
built. (libevent2-openssl and libevent2-pthreads need to be selected for
the necessary functionality to be enabled when compiling libevent2.)

This also adds openssl as an explicit dependency; it appears to be used
directly by this package.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>