doc: add spa to doxygen output

This requires a helper script: doxygen doesn't differ between static methods
and static inline methods. EXTRACT_STATIC defines whether it parses *any*
static method but we're currently using all C files as input files as well. We
cannot convince doxygen to just parse static inline functions in header files
so for SPA we hack around this: meson passes the spa headers to a shell script
with simply copies those changed to `/* static */ inline void (foo)` and doxygen
then runs on those header files.

The result: we get all spa functions added to your doxygen output at the cost
of a few sed calls.

spa/include/meson.build

@@ -1,3 +1,17 @@
+spa_sections = [
+  'buffer',
+  'control',
+  'debug',
+  'graph',
+  'monitor',
+  'node',
+  'param',
+  'pod',
+  'support',
+  'utils',
+]
+
+spa_headers = 'spa'  # used by doxygen
 install_subdir('spa',
   install_dir : get_option('includedir') / spa_name
 )