toolchain: limit musl/kernel headers conflict workaround

toolchain: limit musl/kernel headers conflict workaround

The musl/kernel headers workaround was added in commit 196932cd91
(toolchain: workaround musl/kernel headers conflict) to fix definition
collisions in networking related headers between musl headers and kernel
headers. Kernel headers from version 4.15 and newer do not need this
workaround anymore since kernel commit c0bace798436bc (uapi libc compat:
add fallback for unsupported libcs). The C library does not have to
define the __GLIBC__ macro to make the __UAPI_DEF_* macros effective.

Updated the comment to accordingly.

Tested with the xl2tp package. This package fails to build with older
kernel headers without the workaround (struct in_pktinfo redefinition,
among others). With 4.15 headers, xl2tp builds fine with this patch
applied. That is, no workaround needed.

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