[doc] fix header file paths for out-of-source builds

doc/doxygen/Doxyfile.in

@@ -678,8 +678,8 @@ WARN_LOGFILE           =
 # with spaces.
 
 INPUT                  = . \
-                         ../../RtAudio.h \
-                         ../../RtError.h
+                         @top_srcdir@/RtAudio.h \
+                         @top_srcdir@/rtaudio_c.h
 
 # This tag can be used to specify the character encoding of the source files 
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is