Fix two pointer signedness warnings

Fix two pointer signedness warnings

This fixes the following compiler warnings:

freeradius-server-2.2.6/src/modules/rlm_eap/types/rlm_eap_tls/rlm_eap_tls.c: scope_hint: In function 'cbtls_verify'
freeradius-server-2.2.6/src/modules/rlm_eap/types/rlm_eap_tls/rlm_eap_tls.c:711: warning: pointer targets in passing argument 2 of 'pairmake' differ in signedness
freeradius-server-2.2.6/src/include/libradius.h:373: note: expected 'const char *' but argument is of type 'unsigned char *'

freeradius-server-2.2.6/src/modules/rlm_expr/rlm_expr.c: scope_hint: In function 'base64_to_hex_xlat'
freeradius-server-2.2.6/src/modules/rlm_expr/rlm_expr.c:678: warning: pointer targets in passing argument 1 of 'fr_bin2hex' differ in signedness
freeradius-server-2.2.6/src/include/libradius.h:418: note: expected 'const uint8_t *' but argument is of type 'char *'