Add stdplus dependency for pldm

pldm flight recorder feature in pldm uses stdplus::signal::block
API to block the SIGUSR1 linux signal.

Explicitly not adding this as dependency would ideally break the
compilation of pldm using subprojects.

Tested By:
1. compile pldm on ubuntu with out any openbmc - packages
2. the compilation breaks with undefined reference error
3. Put this patch in - and then recompile the code
4. now pldm compiles with out any errors.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I111f3581b898c71a5e4eeee24a0bfc89c4487021
diff --git a/meson.build b/meson.build
index d86b450..4240916 100644
--- a/meson.build
+++ b/meson.build
@@ -71,6 +71,7 @@
   phosphor_dbus_interfaces = dependency('phosphor-dbus-interfaces')
   sdbusplus = dependency('sdbusplus')
   sdeventplus = dependency('sdeventplus')
+  stdplus = dependency('stdplus')
 
   if cpp.has_header('nlohmann/json.hpp')
     nlohmann_json = declare_dependency()
@@ -162,6 +163,7 @@
   libpldmutils,
   nlohmann_json,
   sdbusplus,
+  stdplus,
   sdeventplus,
   phosphor_dbus_interfaces,
 ]