build: Target c++ arguments specifically where they are needed

This fixes the build with phal enabled, since it tries to add global
project cpp_args after targets have already been declared.

Change-Id: I907fbd40a82725da4e751b82c66c71c602dd2018
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index 7e3099c..68db1cc 100644
--- a/meson.build
+++ b/meson.build
@@ -133,6 +133,7 @@
 
 log_manager_ext_sources = []
 log_manager_ext_deps = []
+log_manager_ext_args = []
 
 subdir('extensions')
 subdir('phosphor-rsyslog-config')
@@ -165,6 +166,7 @@
     log_manager_ext_sources,
     'log_manager_main.cpp',
     include_directories: include_directories('gen'),
+    cpp_args: log_manager_ext_args,
     dependencies: [
         log_manager_deps,
         log_manager_ext_deps,