curl_multibyte: fix compiler error

While compiling lib/curl_multibyte.c with '-DUSE_WIN32_IDN' etc. I was
getting:

f:\mingw32\src\inet\curl\lib\memdebug.h(38): error C2054: expected '('
to follow 'CURL_EXTERN'

f:\mingw32\src\inet\curl\lib\memdebug.h(38): error C2085:
'curl_domalloc': not in formal parameter list