There are several issues here:
1) Each prebuilt static library should have its own module definition, i.e. your Android.mk should look like:
include $(CLEAR_VARS)
LOCAL_MODULE := libpj
LOCAL_SRC_FILES := libpj-arm-unknown-linux-androideabi.a
include $(PREBUILT_STATIC_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := libpjlib-util
LOCAL_SRC_FILES := libpjlib-util-arm-unknown-linux-androideabi.a
include $(PREBUILT_STATIC_LIBRARY)
... etc
Note that the module name doesn't have to be exactly the same as the file path, which makes things a little clearer for the rest.
2) That '.' will refer, at build time, to the current directory where you invoke ndk-build, you need to use $(LOCAL_PATH)/include instead to refer to the right include path (assuming your Android.mk is under jni/ too). I.e.:
include $(CLEAR_VARS)
LOCAL_MODULE := dummy_pjsip_jni_module
LOCAL_SRC_FILES := dummy_pjsua.c
LOCAL_EXPORT_C_INCLUDES += $(LOCAL_PATH)/include
LOCAL_C_INCLUDES += $(LOCAL_PATH)/include
LOCAL_STATIC_LIBRARIES := libpj libpjlib-util libpjmedia libpjmedia-audiodev \
libpjmedia-codec libpjmedia-videodev libpjnath \
libpjsdp libpjsip libpjsip-simple libpjsip-ua \
libpjsua2 libpjsua
include $(BUILD_SHARED_LIBRARY)