package/bearssl: fix BR2_SHARED_STATIC_LIBS case

Reported-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/bearssl/bearssl.mk

@@ -15,11 +15,11 @@ BEARSSL_MAKE_OPTS = \
 	LDDLL=$(TARGET_CC)
 
 ifeq ($(BR2_SHARED_LIBS)$(BR2_SHARED_STATIC_LIBS),y)
-BEARSSL_TARGETS = dll
+BEARSSL_TARGETS += dll
 endif
 
 ifeq ($(BR2_STATIC_LIBS)$(BR2_SHARED_STATIC_LIBS),y)
-BEARSSL_TARGETS = lib
+BEARSSL_TARGETS += lib
 endif
 
 define BEARSSL_BUILD_CMDS