Fix compiler warnings when compiled against musl libc

Fix compiler warnings when compiled against musl libc

musl libc is complaining when <sys/poll.h> is used instead of <poll.h>
so change this.

This issue was reported for OpenWrt/LEDE where musl libc is the standard
C library instead of e.g. glibc, see the following link for the original PR:
https://github.com/openwrt/packages/pull/4263

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Michael Heimpold <mhei@heimpold.de>
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v3: refined checks/fallback paths as suggested by @bukka

v2: rebased to resolve merge conflict in main/php_network.h

v1: initial PR