qemu: workaround 'struct sysinfo' conflict musl <--> linux

Most everyone using musl patches the linux headers instead,
but various software uses a local workaround like the
one added in this commit (psutils, for example).

It's not obvious to me which project has the "bug",
and I'm reluctant to even propose modifying our headers
without clear answer on the issue.

Also, modifying those headers triggers rebuild-all-the-things.

Hopefully upstream projects sort this out, in the meantime
adding this define is a bit of a kludge but does the job.

-------

For the curious, the patch usually is something like this:
https://patchwork.kernel.org/patch/3833241/

Here's an updated version that also ensures
kernel users get the sysinfo struct as expected too:
https://raw.githubusercontent.com/openwrt/openwrt/e3c43ade0bae9491aeea50fa361e846bb5002dc0/target/linux/generic/pending-4.14/270-uapi-kernel.h-glibc-specific-inclusion-of-sysinfo.h.patch

(cherry picked from commit dtzWill/nixpkgs@91b5f5a463)

pkgs/applications/virtualization/qemu/default.nix

@@ -95,6 +95,8 @@ stdenv.mkDerivation rec {
 
   preConfigure = ''
     unset CPP # intereferes with dependency calculation
+  '' + optionalString stdenv.hostPlatform.isMusl ''
+    NIX_CFLAGS_COMPILE+=" -D_LINUX_SYSINFO_H"
   '';
 
   configureFlags =