php7-pecl-krb5: fix build error and runtime loading

php7-pecl-krb5: fix build error and runtime loading

The build throws several (non-fatal) build errors:
-snip-
...
checking for kerberos support... yes, shared
checking path to krb5config tool... no
checking for kerberos KADM5 support... no
checking whether we have krb5config...
./configure: line 3974: --libs: command not found
./configure: line 3975: --cflags: command not found
checking for required linker flags...
checking for required compiler flags...
./configure: line 3988: --version: command not found
checking for kerberos library version...
checking for krb5_free_string... no
checking for krb5_chpw_message... no
checking for krb5_principal_get_realm... no
checking how to print strings... printf
...
-snap-

To fix this give configure the path to krb5-config tool.

The runtime error which is fixed by the change is:
PHP Warning:  PHP Startup: Unable to load dynamic library 'krb5.so'
 (tried: /usr/lib/php/krb5.so (Error relocating
 /usr/lib/php/krb5.so: krb5_get_init_creds_opt_free: symbol not found),
 ...

Signed-off-by: Michael Heimpold <mhei@heimpold.de>