graphics/libgphoto2: Fix curl detection

Before:
checking for libcurl to use... autodetect
checking for LIBCURL... no
checking libcurl config program... none
...

> Parsing plist

> Checking for items in STAGEDIR missing from pkg-plist

> Checking for items in pkg-plist which are not in STAGEDIR

Error: Missing: lib/libgphoto2/%%VERSION%%/lumix.so

> Error: Plist issues found.

• Error code 1

After:
checking for curl/curl.h... yes
checking for function curl_easy_init in libcurl... yes
checking libcurl library flags... "-L/usr/local/lib -lcurl"
checking libcurl cpp flags... "-I/usr/local/include"

Approved by: portmgr (blanket)