misc: Fix BaseCPU doxygen

Doxygen was stopping with #include "arch/null/cpu_dummy.hh" so the html
for BaseCPU was only the three functions defined in the dummy CPU. This
forces doxygen to skip this #include correctly.

Note: The file references for base_cpu still aren't quite right, but it's
better than it was.

Change-Id: Ifafe247df2511caee2569d534bd29348a5ce9e8e
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/11649
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
diff --git a/src/Doxyfile b/src/Doxyfile
index 5056c4d..891720a 100644
--- a/src/Doxyfile
+++ b/src/Doxyfile
@@ -1284,7 +1284,8 @@
 # or name=definition (no spaces). If the definition and the = are 
 # omitted =1 is assumed.
 
-PREDEFINED             = DOXYGEN_SHOULD_SKIP_THIS
+PREDEFINED             = DOXYGEN_SHOULD_SKIP_THIS \
+                         THE_ISA
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then 
 # this tag can be used to specify a list of macro names that should be expanded.