[doc] make doxygen search for input recursively

If RECURSIVE is set to NO doxygen will only process files in emper/
when set to YES it will see all files reachable from emper/.

doc/Doxyfile.in

@@ -908,7 +908,7 @@ FILE_PATTERNS          = *.c \
 # be searched for input files as well.
 # The default value is: NO.
 
-RECURSIVE              = NO
+RECURSIVE              = YES
 
 # The EXCLUDE tag can be used to specify files and/or directories that should be
 # excluded from the INPUT source files. This way you can easily exclude a