musl: add upstream patch to fix race condition in file locking

musl: add upstream patch to fix race condition in file locking

According to [1]:

  musl 1.1.20 introduced a regression in stdio FILE locking that can
  cause soft deadlocks with >2 threads contending for a
  FILE. Users/dists should apply
  https://git.musl-libc.org/cgit/musl/commit/?id=0db393d3a77bb9f300a356c6a5484fc2dddb161d.

[1] https://twitter.com/musllibc/status/1042292786568024070

This commit therefore adds the corresponding patch to the musl
package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>