createrepo-c: Fix function declaration bug found with clang

createrepo-c: Fix function declaration bug found with clang

clang is stricter about function parameter types in its functions and
errors out.

error: incompatible integer to pointer conversion initializing 'gchar *' (aka 'char *')

Real problem is in createrepo_c code where funciton definition and
declaration scopes are different

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>