devshell: include PATH for qt5 native applications

If you perform a "bitbake -c devshell <some qt5 recipe>" and then try "which
qmake" in the devshell, you'll find your host's qmake is found (which is most
likely to be based on qt4) rather than the one that is built as part of
meta-qt5. The reason for this is that the qt5 items are installed to
${STAGING_DIR_NATIVE}/usr/bin/qt5 and not ${STAGING_DIR_NATIVE}/usr/bin. The
reason for the extra "qt5" directory at the end is so that qt4-based items
that are built in OE can live side-by-side with things from meta-qt5 without
interfering with each other.

This patch prepends ${STAGING_DIR_NATIVE}/usr/bin/qt5 to the devshell's PATH
so that the qt5-based native tools which are built as part of meta-qt5 will be
found before the host's.

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

recipes-qt/qt5/qt5.inc

@@ -126,3 +126,5 @@ FILES_${PN}-examples-staticdev = " \
     ${OE_QMAKE_PATH_EXAMPLES}/*/*/*/*/*.a \
     ${OE_QMAKE_PATH_EXAMPLES}/*/*/*/*/*/*.a \
 "
+
+PATH_prepend = "${STAGING_DIR_NATIVE}${OE_QMAKE_PATH_QT_BINS}:"