mxml: Build in source directory

Fix the error below.

    config.status: creating config.h
    [BEE] <= leaving bee_configure() ..
    [BEE] => entering bee_build() ..
    [BEE] make -j1
    make: *** No rule to make target 'mxml-attr.c', needed by 'mxml-attr.o'.  Stop.
    Command exited with non-zero status 2

mxml.be0

@@ -22,7 +22,7 @@ SRCURL[0]="https://github.com/michaelrsweet/mxml/releases/download/v${PKGVERSION
 ## outside the source directory and needs to be built inside the source
 ## directory.
 
-# build_in_sourcedir
+build_in_sourcedir
 
 ###############################################################################
 ## bee cannot detect buildtypes specified in subdirectories.