Revert "tools/mkallsyms.sh: Disable the mismatch warning of builtin declaration"

This reverts commit 848d1ef0b7.

It doesn't make sense because the generated file just have names of
symbols as strings. Nothing to worry about builtins.

tools/mkallsyms.sh

@@ -50,10 +50,6 @@ fi
 echo "#include <nuttx/compiler.h>"
 echo "#include <nuttx/symtab.h>"
 echo ""
-echo "#if defined(__GNUC__) && !defined(__clang__)"
-echo "#  pragma GCC diagnostic ignored \"-Wbuiltin-declaration-mismatch\""
-echo "#endif"
-echo ""
 echo "${CONST} int             g_nallsyms = ${count} + 2;"
 echo "${CONST} struct symtab_s g_allsyms[${count} + 2] = "
 echo "{"