clangd tells me that the gdb_signals.h include in target/waitstatus.h is
unused. This include was probably to give access to `enum gdb_signal`,
but this is in fact defined in gdb/signals.h. Change the include to
gdb/signals.h. Include gdbsupport/gdb_signals.h in some files that were
relying on the transitive include.
Change-Id: I6f4361b3d801394bf29abe8c1393aff110aa0ad6
