libc: Provide both getpgid and __getpgid symbols
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 <532378793705a04edd56deb76ad8c0442834d55d@st.com>

skristiansson/uClibc-or1k,hwoarang/uClibc,brgl/uclibc-ng,kraj/uClibc,ffainelli/uClibc,majek/uclibc-vx32,m-labs/uclibc-lm32,foss-xtensa/uClibc,kraj/uclibc-ng,ndmsystems/uClibc,groundwater/uClibc,czankel/xtensa-uclibc,ffainelli/uClibc,waweber/uclibc-clang,wbx-github/uclibc-ng,skristiansson/uClibc-or1k,ffainelli/uClibc,skristiansson/uClibc-or1k,mephi42/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,czankel/xtensa-uclibc,ysat0/uClibc,ddcc/klee-uclibc-0.9.33.2,foss-xtensa/uClibc,brgl/uclibc-ng,wbx-github/uclibc-ng,brgl/uclibc-ng,hjl-tools/uClibc,kraj/uclibc-ng,atgreen/uClibc-moxie,ffainelli/uClibc,mephi42/uClibc,mephi42/uClibc,majek/uclibc-vx32,m-labs/uclibc-lm32,hwoarang/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,hwoarang/uClibc,hjl-tools/uClibc,ysat0/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,kraj/uclibc-ng,m-labs/uclibc-lm32,wbx-github/uclibc-ng,groundwater/uClibc,hwoarang/uClibc,czankel/xtensa-uclibc,atgreen/uClibc-moxie,waweber/uclibc-clang,m-labs/uclibc-lm32,brgl/uclibc-ng,ddcc/klee-uclibc-0.9.33.2,foss-for-synopsys-dwc-arc-processors/uClibc,hjl-tools/uClibc,groundwater/uClibc,kraj/uClibc,czankel/xtensa-uclibc,waweber/uclibc-clang,groundwater/uClibc,ndmsystems/uClibc,atgreen/uClibc-moxie,ysat0/uClibc,ddcc/klee-uclibc-0.9.33.2,skristiansson/uClibc-or1k,mephi42/uClibc,hjl-tools/uClibc,hjl-tools/uClibc,kraj/uclibc-ng,ndmsystems/uClibc,atgreen/uClibc-moxie,majek/uclibc-vx32,groundwater/uClibc,kraj/uClibc,ndmsystems/uClibc,majek/uclibc-vx32,waweber/uclibc-clang,kraj/uClibc,ffainelli/uClibc,foss-xtensa/uClibc,wbx-github/uclibc-ng,foss-xtensa/uClibc,ddcc/klee-uclibc-0.9.33.2,ysat0/uClibc