kselftests: re-add ptp into main RDEPENDS

Commit 371b7bb522 enabled ptp package, but
did not add it to RDEPENDS, fix it.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>

recipes-kernel/kselftests/kselftests_4.10.bb

@@ -160,6 +160,7 @@ RDEPENDS_${PN} += "bash \
 	${PN}-net \
 	${PN}-nsfs \
 	${PN}-pstore \
+	${PN}-ptp \
 	${PN}-ptrace \
 	${PN}-seccomp \
 	${PN}-sigaltstack \