meson.build: bump Meson minimum version to 0.59.0

When creating a new build directory, Meson prints a warning:
  WARNING: Project specifies a minimum meson_version '>= 0.56.0' but
  uses features which were added in newer versions:
   * 0.59.0: {'unescaped_variables arg in pkgconfig.generate'}

Since 0.59.0 was released a while ago and no one has complained so far,
it should be reasonable to just bump the minimum version required,
rather than try to make it 0.56.0 compliant.

meson.build

@@ -1,7 +1,7 @@
 project('pipewire', ['c' ],
   version : '0.3.43',
   license : [ 'MIT', 'LGPL-2.1-or-later', 'GPL-2.0-only' ],
-  meson_version : '>= 0.56.0',
+  meson_version : '>= 0.59.0',
   default_options : [ 'warning_level=3',
                       'c_std=gnu99',
                       'cpp_std=c++17',