build: rename dependency variable

Per [1], the conventions for dependency variables are to
be named '<project or lib>_dep'.  This allows consistency
when using this project as a meson-subproject elsewhere.

Switch the name from 'libpldm' to 'libpldm_dep'.

1. https://mesonbuild.com/Subprojects.html#naming-convention-for-dependency-variables



Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I98c8e2148610b45755b45029d1d38206bac5b885