Change around the header install path

This installs the headers to the same place, but for some reason
something on a2 isn't working correctly and is trying to remove the
headers from /usr first.

Makefile.in

@@ -143,14 +143,14 @@ stamps/build-glibc-linux-headers: src/glibc stamps/build-gcc-linux-stage1
 	mkdir -p $(SYSROOT)/usr/lib $(SYSROOT)/lib
 	cd $(notdir $@) && CC= CXX= $(CURDIR)/$</configure \
 		--host=riscv$(XLEN)-unknown-linux-gnu \
-		--prefix=/usr \
+		--prefix=$(SYSROOT)/usr \
 		libc_cv_forced_unwind=yes \
 		libc_cv_c_cleanup=yes \
 		--enable-shared \
 		--enable-__thread \
 		--disable-multilib \
 		--enable-kernel=2.6.32
-	$(MAKE) -C $(notdir $@) install-headers install_root=$(SYSROOT)
+	$(MAKE) -C $(notdir $@) install-headers
 	mkdir -p $(dir $@) && touch $@
 
 stamps/build-glibc-linux$(XLEN)$(MULTILIB): src/glibc stamps/build-gcc-linux-stage1