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.

doc/Doxyfile.in

@@ -14,6 +14,7 @@ QUIET                  = YES
 WARN_NO_PARAMDOC       = YES
 INPUT                  = @inputs@
 FILE_PATTERNS          = "*.h" "*.c"
+RECURSIVE              = YES
 EXAMPLE_PATH           = "@top_srcdir@/src/tools" \
                          "@top_srcdir@/src/examples"