package/git-crypt: fix build with gcc-4.9

Fixes

In file included from commands.cpp:32:0:
crypto.hpp:60:7: error: 'unique_ptr' in namespace 'std' does not name a template type
  std::unique_ptr<Aes_impl> impl;

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/git-crypt/git-crypt.mk

@@ -14,7 +14,8 @@ GIT_CRYPT_LIBS = `$(PKG_CONFIG_HOST_BINARY) --libs openssl`
 
 define GIT_CRYPT_BUILD_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D) $(TARGET_CONFIGURE_OPTS) \
-		LDFLAGS="$(TARGET_LDFLAGS) $(GIT_CRYPT_LIBS)" PREFIX=/usr
+		LDFLAGS="$(TARGET_LDFLAGS) $(GIT_CRYPT_LIBS)" \
+		CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11" PREFIX=/usr
 endef
 
 define GIT_CRYPT_INSTALL_TARGET_CMDS