Exclude amp_conf.h from Doxygen processing

As amp_conf.h only contains private function declarations exclude it
from Doxygen processing for now.

doc/hamlib.cfg.in

@@ -21,6 +21,8 @@ INPUT            = @top_srcdir@/doc/index.doxygen \
                     @top_srcdir@/include/hamlib/ \
                     @top_srcdir@/src/
 
+EXCLUDE          = @top_srcdir@/src/amp_conf.h
+
 INCLUDE_PATH     = @top_srcdir@/include
 
 EXAMPLE_PATH     = @top_srcdir@/tests/testrig.c \