contrib: fluid: build with PIC

I got a Linux x86-64 build complaining it was not built with PIC.

/usr/bin/ld: /root/build/contrib/x86_64-linux-gnu/lib/libfluidsynth.a(fluid_sys.c.o): warning: relocation against `in6addr_any@@GLIBC_2.2.5' in read-only section `.text'
/usr/bin/ld: /root/build/contrib/x86_64-linux-gnu/lib/libfluidsynth.a(fluid_sys.c.o): relocation R_X86_64_PC32 against symbol `stderr@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC

contrib/src/fluid/rules.mak

@@ -48,7 +48,7 @@ endif
 
 .fluid: fluidsynth toolchain.cmake
 	$(CMAKECLEAN)
-	$(HOSTVARS) $(CMAKE) $(FLUIDCONF)
+	$(HOSTVARS) $(CMAKE_PIC) $(FLUIDCONF)
 	+$(CMAKEBUILD)
 	$(CMAKEINSTALL)
 	touch $@