package/xml-security-c: fix syntax error

package/xml-security-c: fix syntax error

Add missed backslash to fix such error:
package/xml-security-c/Config.in:16: syntax error
package/xml-security-c/Config.in:15: invalid option
package/xml-security-c/Config.in:15:warning: ignoring unsupported character '!'
package/xml-security-c/Config.in:16: unknown option "BR2_TOOLCHAIN_GCC_AT_LEAST_4_7"

Fixes: bbda38afbbfa ("package/xml-security-c: new package")
Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>