squeezelite: fix musl build

The build error was not yet found by the autobuilders:

output_alsa.c: In function ‘output_init_alsa’:
output_alsa.c:865:10: error: ‘M_TRIM_THRESHOLD’ undeclared (first use in this function)
  mallopt(M_TRIM_THRESHOLD, -1);
          ^
output_alsa.c:865:10: note: each undeclared identifier is reported only once for each function it appears in
output_alsa.c:866:10: error: ‘M_MMAP_MAX’ undeclared (first use in this function)
  mallopt(M_MMAP_MAX, 0);
          ^

This commit adds a patch fixing this build error by making the
mallopt() usage conditional on __GLIBC__.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: only enclose the mallopt() call in __GLIBC__, use Git to
format the patch, improve the commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>