configure: check for declaration of getpwuid_r

On our x86 Android toolchain, getpwuid_r is implemented but the header
is missing:

 netrc.c:81:7: error: implicit declaration of function 'getpwuid_r' [-Werror=implicit-function-declaration]

Unfortunately, the function is used in curl_ntlm_wb.c, too, so I moved
the prototype to curl_setup.h.

Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Closes #2609