Fix build error when building an app with libcper as a subproject

Addresses the following error from meson setup:
subprojects/libcper/meson.build:15:0: ERROR: Function
'add_global_arguments' cannot be used in subprojects because there
is no way to make that reliable.

Change-Id: I69f42e19c0c8cbc13e21dbee2848275effc65e45
Signed-off-by: Karthik Rajagopalan <krajagopalan@nvidia.com>
diff --git a/meson.build b/meson.build
index 865f50a..1e62076 100644
--- a/meson.build
+++ b/meson.build
@@ -12,7 +12,7 @@
     ],
 )
 
-add_global_arguments(
+add_project_arguments(
     '-DLIBCPER_JSON_SPEC="'
     + meson.current_source_dir()
     + '/specification/json/cper-json.json"',
@@ -20,7 +20,7 @@
 )
 
 library_is_share = get_option('default_library') == 'shared'
-add_global_arguments('-D_POSIX_C_SOURCE=200809L', language : 'c')
+add_project_arguments('-D_POSIX_C_SOURCE=200809L', language : 'c')
 
 project_description = 'libcper library'