meson: Define LIBPLDM_ABI_DEPRECATED_UNSAFE as empty as required

Not doing so for -Dabi=stable,testing causes unnecessary compilation
errors. The library now builds with:

    meson setup -Dabi=stable,testing -Dtests=false ...

Suggested-by: Sam Abu-Nassar <sam.abunassar@xconn-tech.com>
Fixes: 36324f6bea8c ("Apply GCC's tainted_args attribute to library entrypoints")
Change-Id: Ie825e036171edeec321c4df244c075da288b60e4
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 2ed66e7..6f2b11f 100644
--- a/meson.build
+++ b/meson.build
@@ -59,6 +59,7 @@
     add_project_arguments('-DLIBPLDM_API_DEPRECATED', language: ['c', 'cpp'])
 else
     conf.set('LIBPLDM_ABI_DEPRECATED', '')
+    conf.set('LIBPLDM_ABI_DEPRECATED_UNSAFE', '')
 endif
 conf.set('LIBPLDM_ABI_STABLE', entrypoint)  # Always expose the stable symbols
 if get_option('abi').contains('testing')