package/sngrep: fix build with musl libc

package/sngrep: fix build with musl libc

Add upstream patch fixing typo that causes build failure under musl,
where stdin is constant.

Fixes:
http://autobuild.buildroot.net/results/ab827021d4dae8ae98f667328e4a9a7e1c4288d0/
http://autobuild.buildroot.net/results/e99e937c05be38d33e029fbfc737665b590c33bd/
http://autobuild.buildroot.net/results/e8ca0a40d8cd2c333b55fa9e67ddeead1f502d5a/

Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>