docs: Update BUILDING.md

Indicate that GNU Make is the version of make required to build.

BUILDING.md

@@ -19,6 +19,7 @@ LLVM, Clang and LLD are quite light on dependencies, requiring only standard
 build tools to be built. Go is of course necessary to build TinyGo itself.
 
   * Go (1.18+)
+  * GNU Make
   * Standard build tools (gcc/clang)
   * git
   * CMake