Add an include path to the doxyfile to fix preprocessing for docs

Previously, doxygen was unable to effectively use preprocessing to determine
which parts of code should be documented. This was due to its inability to
include headers during preprocessing, which was discovered by running it in
preprocessing debug mode:
doxygen -d preprocessor mbedtls.doxyfile 
An example of failure: "#include mbedtls/config.h: not found! skipping..."
With the following fix includes are properly preprocessed with the documentation
being considerably larger.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>

doxygen/mbedtls.doxyfile

@@ -1594,7 +1594,7 @@ SEARCH_INCLUDES        = YES
 # contain include files that are not input files but should be processed by
 # the preprocessor.
 
-INCLUDE_PATH           =
+INCLUDE_PATH           = ../include
 
 # You can use the INCLUDE_FILE_PATTERNS tag to specify one or more wildcard
 # patterns (like *.h and *.hpp) to filter out the header-files in the