meson: Use __attribute__ for consistency

While gcc accepts __attribute as an alternative spelling, use
__attribute__ for consistency and to align with common documentation.
Use of __attribute wasn't intentional to begin with, just too much
doubling up of characters everywhere.

Change-Id: I81eb8fc0ae3952c15efd180c4b8286671516135b
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/meson.build b/meson.build
index 9d5a60b..2ed66e7 100644
--- a/meson.build
+++ b/meson.build
@@ -54,7 +54,7 @@
     conf.set('LIBPLDM_ABI_DEPRECATED', entrypoint)
     conf.set(
         'LIBPLDM_ABI_DEPRECATED_UNSAFE',
-        '__attribute((visibility("default")))',
+        '__attribute__((visibility("default")))',
     )
     add_project_arguments('-DLIBPLDM_API_DEPRECATED', language: ['c', 'cpp'])
 else