gd/parser: Fix flaky compilation

When compiling net-wireless/floss, flex_source sometimes complains include
file generated by bison_source (language_y.h) is missing.

Address the issue by enforcing dependency of bison on flex.

Bug: 211658554
Tag: #stability
Test: git am changes in chromeos aosp/ packages/modules/Bluetooth/local
Compile floss with USE="-floss_upstream -cros-debug" emerge-dedede floss
Check changes is taken into account and does not break compilation.

Signed-off-by: Gwendal Grignou <gwendal@google.com>
Change-Id: I52f4752268a48640acad9bb8e5da2bbeaced102a