configure: create fsdev/ directory

In some cases when building with parallelism (make -jN),
build fails because the directory where output files are
supposed to be does not exist.  In particular, when make
decides to build virtfs-proxy-helper.1 before other files
in fsdev/, build will fail with the following error:

perl -Ww -- BUILDDIR/scripts/texi2pod.pl BUILDDIR/fsdev/virtfs-proxy-helper.texi fsdev/virtfs-proxy-helper.pod && pod2man --utf8 --section=1 --center=" " --release=" " fsdev/virtfs-proxy-helper.pod > fsdev/virtfs-proxy-helper.1
opening "fsdev/virtfs-proxy-helper.pod": No such file or directory
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -4647,6 +4647,7 @@ fi
 
 # build tree in object directory in case the source is not in the current directory
 DIRS="tests tests/tcg tests/tcg/cris tests/tcg/lm32 tests/libqos tests/qapi-schema tests/tcg/xtensa tests/qemu-iotests"
+DIRS="$DIRS fsdev"
 DIRS="$DIRS pc-bios/optionrom pc-bios/spapr-rtas pc-bios/s390-ccw"
 DIRS="$DIRS roms/seabios roms/vgabios"
 DIRS="$DIRS qapi-generated"