fix: EDK2 BaseTools build with modern GCC

PCCTS parser generator uses K&R-style function pointers void (*)() which
modern GCC (12+) rejects under -Werror (treats () as (void)).
Build with -std=gnu89 -Wno-error for the BaseTools C toolchain.

firmware/build_ovmf.sh

@@ -19,7 +19,11 @@ EDK2_DIR="${EDK2_DIR:-$(pwd)}"
 }
 
 echo "=== Building EDK2 BaseTools (C toolchain) ==="
-make -C BaseTools/Source/C -j"$(nproc)"
+# PCCTS parser code uses K&R-style function pointers (void (*)()) which
+# modern GCC rejects with -Werror. Build with -Wno-error and gnu89.
+make -C BaseTools/Source/C -j"$(nproc)" \
+    CFLAGS="-MMD -MP -g -O2 -Wno-error -std=gnu89" \
+    BUILD_CFLAGS="-MMD -MP -g -O2 -Wno-error -std=gnu89"
 
 echo "=== Initializing EDK2 environment ==="
 # shellcheck disable=SC1091