termios-stub.cpp: make sure _GNU_SOURCE is defined #3264

Job	Run time
Build mlibc (x86_64, mlibc)	3m 45s
Compile sysdeps (dripos)	2m 13s
Build mlibc (x86_64, mlibc-static)	2m 54s
Build mlibc (x86_64, mlibc-shared)	2m 54s
Compile sysdeps (lemon)	2m 20s
Build mlibc (x86_64, mlibc-ansi-only)	2m 58s
Compile sysdeps (aero)	2m 31s
Compile sysdeps (ironclad)	2m 17s
Build mlibc (x86_64, mlibc-headers-only)	1m 52s
Compile sysdeps (lyre)	2m 28s
Build mlibc (riscv64, mlibc)	3m 34s
Compile sysdeps (keyronex)	2m 17s
Build mlibc (riscv64, mlibc-static)	4m 34s
Build mlibc (riscv64, mlibc-shared)	3m 0s
Build mlibc (riscv64, mlibc-ansi-only)	2m 47s
Build mlibc (riscv64, mlibc-headers-only)	1m 48s
Build mlibc (aarch64, mlibc)	3m 55s
Build mlibc (aarch64, mlibc-static)	3m 1s
Build mlibc (aarch64, mlibc-shared)	3m 7s
Build mlibc (aarch64, mlibc-ansi-only)	2m 52s
Build mlibc (aarch64, mlibc-headers-only)	1m 44s
	58m 51s

Provide feedback