meson: Rename the compiler flags variable

Renamed the variable holding the compiler flags to be consistent
with different meson ports.

This naming pattern improves the use of different compiler flags
in environments with multiple languages.

src/dhcp/meson.build

@@ -1,6 +1,6 @@
 name = 'nm-dhcp-helper'
 
-cflags = [
+c_flags = [
   '-DG_LOG_DOMAIN="@0@"'.format(name),
   '-DNETWORKMANAGER_COMPILATION=NM_NETWORKMANAGER_COMPILATION_GLIB',
 ]
@@ -9,7 +9,7 @@ executable(
   name,
   name + '.c',
   dependencies: glib_nm_default_dep,
-  c_args: cflags,
+  c_args: c_flags,
   link_args: ldflags_linker_script_binary,
   link_depends: linker_script_binary,
   install: true,