spa: add a test for missing includes

For each header in the spa directory, generate a compilation test that
includes just that header. This way we can pick up missing #includes.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

spa/tests/spa-include-test-template.c

@@ -0,0 +1,5 @@
+#include <@INCLUDE@>
+
+int main(void) {
+	return 0;
+}