misc: Doxygen tweaks: fixed some typos and minor errors

Doxygen generates quite a lot of warnings on libosmocore. Some of them
are obvious typos - this patch aims to fix such low-hanging fruit.
diff --git a/Doxyfile.gsm.in b/Doxyfile.gsm.in
index 36a6ae2..c4a837f 100644
--- a/Doxyfile.gsm.in
+++ b/Doxyfile.gsm.in
@@ -688,7 +688,7 @@
 # directories that contain image that are included in the documentation (see
 # the \image command).
 
-IMAGE_PATH             = images/
+# IMAGE_PATH             = images/
 
 # The INPUT_FILTER tag can be used to specify a program that doxygen should
 # invoke to filter for each input file. Doxygen will invoke the filter program
@@ -1406,7 +1406,7 @@
 # evaluate all C-preprocessor directives found in the sources and include
 # files.
 
-ENABLE_PREPROCESSING   = NO
+ENABLE_PREPROCESSING   = YES
 
 # If the MACRO_EXPANSION tag is set to YES Doxygen will expand all macro
 # names in the source code. If set to NO (the default) only conditional