package/llvm: fix typo in llvm.mk

"+=" was missing in -DLLVM_TARGETS_TO_BUILD, so LLVM_CONF_OPTS and
HOST_LLVM_CONF_OPTS before this option weren't being considered.

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/llvm/llvm.mk

@@ -40,8 +40,8 @@ LLVM_TARGET_ARCH = $(call qstrip,$(BR2_PACKAGE_LLVM_TARGET_ARCH))
 
 # Build backend for target architecture. This include backends like AMDGPU.
 LLVM_TARGETS_TO_BUILD = $(LLVM_TARGET_ARCH)
-HOST_LLVM_CONF_OPTS = -DLLVM_TARGETS_TO_BUILD="$(subst $(space),;,$(LLVM_TARGETS_TO_BUILD))"
-LLVM_CONF_OPTS = -DLLVM_TARGETS_TO_BUILD="$(subst $(space),;,$(LLVM_TARGETS_TO_BUILD))"
+HOST_LLVM_CONF_OPTS += -DLLVM_TARGETS_TO_BUILD="$(subst $(space),;,$(LLVM_TARGETS_TO_BUILD))"
+LLVM_CONF_OPTS += -DLLVM_TARGETS_TO_BUILD="$(subst $(space),;,$(LLVM_TARGETS_TO_BUILD))"
 
 # LLVM target to use for native code generation. This is required for JIT generation.
 # It must be set to LLVM_TARGET_ARCH for host and target, otherwise we get