package/pppd: fix build with libressl

Fix the following build failure with libressl:

eap-tls.c: In function 'ssl_msg_callback':
eap-tls.c:1284:10: error: 'SSL3_RT_HEADER' undeclared (first use in this function); did you mean 'SSL3_RT_ALERT'?
 1284 |     case SSL3_RT_HEADER:
      |          ^~~~~~~~~~~~~~
      |          SSL3_RT_ALERT

Fixes:
 - http://autobuild.buildroot.org/results/7d721833bddf73531fa03b0a626511af6826d0df



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