package/upx: fix build with host gcc 6.x

Disable the -Werror gcc option because is causes the following error
when using gcc 6.x on the host.

 p_vmlinx.cpp:100:5: error: this ‘if’ clause does not guard...
  [-Werror=misleading-indentation] if (a->p_paddr > b->p_paddr) return  1;

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/upx/upx.mk

@@ -17,6 +17,7 @@ HOST_UPX_DEPENDENCIES = host-ucl host-zlib
 define HOST_UPX_BUILD_CMDS
 	$(HOST_MAKE_ENV) $(MAKE) CPPFLAGS="$(HOST_CPPFLAGS)" \
 		LDFLAGS="$(HOST_LDFLAGS)" UPX_UCLDIR=$(HOST_DIR)/usr \
+		CXXFLAGS_WERROR= \
 		-C $(@D) all
 endef