gatling: decrease optimisation

When compiling on MIPS-architectures, in -O2 there happen some
errors on interlinking. Linker will will fail with:

```
Unsupported jump between ISA modes; consider recompiling with interlinking enabled
```

Reducing optimisation works around these errors and looks like
a fairly common solution to this problem.
https://forum.openwrt.org/t/unsupported-jump-between-isa-modes-consider-recompiling-with-interlinking-enabled/55786

Signed-off-by: Martin Hübner <martin.hubner@web.de>
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -47,7 +47,7 @@ ifneq ($(DEBUG),)
 CFLAGS+=-g -Og
 LDFLAGS+=-g
 else
-CFLAGS+=-O2 -fomit-frame-pointer -I/usr/local/include
+CFLAGS+=-O1 -fomit-frame-pointer -I/usr/local/include
 LDFLAGS+=-s
 ifneq ($(DIET),)
 DIET+=-Os