erlang-p1-stringprep: backport upstream commit to fix musl build

Amusingly, a fix made upstream to fix the build on Windows also fixes
the build with the musl C library.

Fixes:

  http://autobuild.buildroot.net/results/d7b0cacc5d99d8dc91fdeaf770bb5d2b1b1975b6/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>