build: Fix FTBFS using meson without valgrind

Now with meson build system libsecret FTBFS without valgrind installed.
Two issues: (1) egg-testing.c depends on valgrind/valgrind.h unnecessarily;
(2) we forgot to add "build" directory as a header directory for meson.

This commit resolves both of them.

egg/egg-testing.c

@@ -27,8 +27,6 @@
 
 #include <glib-object.h>
 
-#include <valgrind/valgrind.h>
-
 #include <glib.h>
 #include <glib/gstdio.h>
 

egg/meson.build

@@ -21,5 +21,5 @@ endif
 libegg = static_library('egg',
   libegg_sources,
   dependencies: libegg_deps,
-  include_directories: config_h_dir,
+  include_directories: [config_h_dir, build_dir],
 )

meson.build

@@ -24,6 +24,7 @@ with_gtkdoc = get_option('gtk_doc')
 
 # Some variables
 config_h_dir = include_directories('.')
+build_dir = include_directories('build')
 libsecret_prefix = get_option('prefix')
 datadir = get_option('datadir')
 includedir = get_option('includedir')