Doxyfile: Set STRIP_FROM_INC_PATH to project root

Show proper #include's on struct pages.

Before:

    #include <buffer.h>

After

    #include <libavutil/buffer.h>

doc/Doxyfile

@@ -161,7 +161,7 @@ STRIP_FROM_PATH        = .
 # specify the list of include paths that are normally passed to the compiler
 # using the -I flag.
 
-STRIP_FROM_INC_PATH    =
+STRIP_FROM_INC_PATH    = .
 
 # If the SHORT_NAMES tag is set to YES, doxygen will generate much shorter (but
 # less readable) file names. This can be useful is your file systems doesn't