Doxygen: Increase DOT_GRAPH_MAX_NODES to 120

This avoids a currently happening error:

"error: Include graph for 'src_ipc4_int32_table.h' not generated,
too many nodes (89), threshold is 50. Consider increasing
DOT_GRAPH_MAX_NODES."

and another warning that happened with first increase to 100:

"warning: Included by graph for 'src.h' not generated, too many
nodes (111), threshold is 100. Consider increasing
DOT_GRAPH_MAX_NODES."

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

doc/sof.doxygen.in

@@ -39,3 +39,6 @@ TYPEDEF_HIDES_STRUCT = YES
 #FILTER_SOURCE_FILES = YES
 
 HTML_TIMESTAMP = NO
+
+# Avoid error "Include graph for 'foobar.h' not generated, too many nodes"
+DOT_GRAPH_MAX_NODES = 120