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
diff --git a/libpldmresponder/meson.build b/libpldmresponder/meson.build
index 4377e39..952b702 100644
--- a/libpldmresponder/meson.build
+++ b/libpldmresponder/meson.build
@@ -2,7 +2,7 @@
   phosphor_dbus_interfaces,
   sdbusplus,
   sdeventplus,
-  libpldm,
+  libpldm_dep,
   libpldmutils
 ]