nptl: fix compiler warning due to missing prototype
libpthread/nptl/sysdeps/pthread/pt-longjmp.c: In function 'longjmp':
libpthread/nptl/sysdeps/pthread/pt-longjmp.c:27:3: warning: implicit declaration of function '__libc_longjmp' [-Wimplicit-function-declaration]
libpthread/nptl/sysdeps/pthread/pt-longjmp.c:28:1: warning: 'noreturn' function does return [enabled by default]
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>