libc: Provide both getpgid and __getpgid symbols

Indeed unistd.h header does export '__getpgid' by default, while
uClibc provide only 'getpgid' implementation. The 'getpgid' symbol
is exported by standard header if __USE_XOPEN_EXTENDED is defined.
This patch alignes uClibc implementation with standard header (matching
with glibc behaviour).

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

libc/sysdeps/linux/common/getpgid.c

@@ -15,8 +15,11 @@
 #define __NR___syscall_getpgid __NR_getpgid
 static __inline__ _syscall1(__kernel_pid_t, __syscall_getpgid, __kernel_pid_t, pid)
 
-pid_t getpgid(pid_t pid)
+pid_t __getpgid(pid_t pid)
 {
 	return (__syscall_getpgid(pid));
 }
+#ifdef __USE_XOPEN_EXTENDED
+weak_alias(__getpgid,getpgid)
+#endif
 #endif