main/linux-grsec: add sed to makedepends

Got the following build error (musl libc):

arch/x86/boot/header.S:425: Error: can't resolve `VO__end' {*UND* section}
  - `VO__text' {*UND* section}

Internet says this is fixed by using GNU sed, and so it was.

main/linux-grsec/APKBUILD

@@ -11,7 +11,7 @@ pkgrel=0
 pkgdesc="Linux kernel with grsecurity"
 url=http://grsecurity.net
 depends="mkinitfs linux-firmware"
-makedepends="perl installkernel bash gmp-dev bc"
+makedepends="perl sed installkernel bash gmp-dev bc"
 options="!strip"
 _config=${config:-kernelconfig.${CARCH}}
 install=