fix for doxygen 1.8.7

Earlier versions apparently used HTML as default if no output format was
specified. 1.8.7 no longer does that.

Change-Id: Ia8f908f54b344530e1b50e8800c032f782fb36e9

odk/docs/cpp/Doxyfile

@@ -24,6 +24,7 @@ WARN_FORMAT = "$file:$line: $text"
 INPUT = %
 RECURSIVE = YES
 
+GENERATE_HTML = YES
 HTML_OUTPUT = .
 
 GENERATE_LATEX = NO

odk/docs/idl/Doxyfile

@@ -13,6 +13,7 @@ STRIP_FROM_PATH = %
 INPUT = %
 RECURSIVE = YES
 
+GENERATE_HTML = YES
 HTML_OUTPUT = .
 GENERATE_LATEX = NO