build: add symlink for pldm.h

'libpldm/requester/pldm.h' is typically installed into
'libpldm/pldm.h' when enabled, but since the file is not
present in this repository it is impossible for other projects
to use pldm both globally and as a meson subproject.  Add a
symlink for this file so that when used as a subproject, the
header can be found with '#include <libpldm/pldm.h>' just as
it would in a global context.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie0a06803a2760de81b8ee250ff4c84b51914f520