util/shell.nix: Explicitly install gnumake and GCC packages

Even if both packages are provided by stdenv, add the packages gnumake
and GCC to buildInputs so that the list of (build-) dependencies is
complete.

Signed-off-by: Felix Singer <felixsinger@posteo.net>
Change-Id: I683eb0459404738a501a57d4301a90a6e3b46f93
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/67087
Original-Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Original-Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73401
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/util/shell.nix b/util/shell.nix
index 2e7094a..c870ebc 100644
--- a/util/shell.nix
+++ b/util/shell.nix
@@ -4,6 +4,8 @@
 	name = "flashrom";
 
 	buildInputs = [
+		gcc
+		gnumake
 		libftdi1
 		libjaylink
 		libusb1