selftests: mptcp: extend CFLAGS to keep options from environment

selftests: mptcp: extend CFLAGS to keep options from environment

Package build environments like Fedora rpmbuild introduced hardening
options (e.g. -pie -Wl,-z,now) by passing a -spec option to CFLAGS
and LDFLAGS.

mptcp Makefile currently overrides CFLAGS but not LDFLAGS, which leads
to a mismatch and build failure, for example:
  make[1]: *** [../../lib.mk:222: tools/testing/selftests/net/mptcp/mptcp_sockopt] Error 1
  /usr/bin/ld: /tmp/ccqyMVdb.o: relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a PIE object; recompile with -fPIE
  /usr/bin/ld: failed to set dynamic section sizes: bad value
  collect2: error: ld returned 1 exit status

Fixes: cc937dad85ae ("selftests: centralize -D_GNU_SOURCE= to CFLAGS in lib.mk")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/7abc701da9df39c2d6cd15bc3cf9e6cee445cb96.1737621162.git.jstancek@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>