package/libsrtp: fix static build

The library name was updated upstream:

Fixes
make[1]: *** No rule to make target 'libsrtp.a'.  Stop.

http://autobuild.buildroot.net/results/10e/10e4198e8d86eecb9c672db77cbaff1e8a592ad3/

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

package/libsrtp/libsrtp.mk

@@ -11,11 +11,11 @@ LIBSRTP_LICENSE = BSD-3-Clause
 LIBSRTP_LICENSE_FILES = LICENSE
 
 ifeq ($(BR2_STATIC_LIBS),y)
-LIBSRTP_MAKE_OPTS = libsrtp.a
+LIBSRTP_MAKE_OPTS = libsrtp2.a
 else ifeq ($(BR2_SHARED_LIBS),y)
 LIBSRTP_MAKE_OPTS = shared_library
 else
-LIBSRTP_MAKE_OPTS = libsrtp.a shared_library
+LIBSRTP_MAKE_OPTS = libsrtp2.a shared_library
 endif
 
 # While libsrtp is not using pkg-config itself, it checks if