linux-purism-librem5dev: set REPLACE_GCCH=0

Do not replace compiler-gcc.h. This is necessary now, as we are
replacing compiler-gcc.h by default again, as it is needed for most
kernels. Not increasing the pkgrel to avoid unnecessary rebuild, I have
verified that this works.

NOTE: in the last commit message, I wrote about OVERWRITE_GCC=0. But
the variable is called REPLACE_GCC, as I've decided to rename it before
submitting. I forgot to change it in the commit message.

[skip ci]: this kernel takes more than one hour to build, so it won't
           finish in CI.

device/linux-purism-librem5dev/APKBUILD

@@ -30,7 +30,8 @@ builddir="$srcdir/${_repository}-${_commit}"
 
 prepare() {
 	default_prepare
-	downstreamkernel_prepare "$srcdir" "$builddir" "$_config" "$_carch" "$HOSTCC"
+	REPLACE_GCCH=0 \
+		downstreamkernel_prepare "$srcdir" "$builddir" "$_config" "$_carch" "$HOSTCC"
 }
 
 build() {