package/tyrian: fixes compilation with static libs

Has been tested with: "./support/scripts/test-pkg -c tyrian.cfg -p opentyrian"
Fixes: http://autobuild.buildroot.net/results/0e2345db82b33f591958fc0f72ad914adafe0522
and some similar previous build failure.

Thanks Thomas for the tip ;-).

Signed-off-by: Julien BOIBESSOT <julien.boibessot@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit dd99f2cb7d)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/opentyrian/opentyrian.mk

@@ -24,7 +24,7 @@ define OPENTYRIAN_BUILD_CMDS
 		CC="$(TARGET_CC)" \
 		STRIP="/bin/true" \
 		SDL_CONFIG="$(STAGING_DIR)/usr/bin/sdl-config" \
-		LDFLAGS="-lm" \
+		LDFLAGS="$(TARGET_LDFLAGS) -lm" \
 		WITH_NETWORK="$(OPENTYRIAN_NETWORK)" \
 		-C $(@D) release
 endef