doc: Correct absolute path in doxygen graphs.

Adds a config item to prevent doxygen from using absolute paths in
generated diagrams. This trims off the base directory during config to
leave just relative paths.

Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

doc/doxygen/tooldev.doxygen.in

@@ -9,3 +9,4 @@ INPUT                  = \
 RECURSIVE              = YES
 GENERATE_LATEX         = NO
 DOTFILE_DIRS           = @top_srcdir@/doc/doxygen
+STRIP_FROM_PATH        = @top_srcdir@

doc/doxygen/tools.doxygen.in

@@ -8,3 +8,4 @@ INPUT                  = \
 		       @top_srcdir@/tools/zunitc/inc/zunitc/zunitc.h
 GENERATE_LATEX         = NO
 DOTFILE_DIRS           = @top_srcdir@/doc/doxygen
+STRIP_FROM_PATH        = @top_srcdir@