devel/cmake-core: Fix build with libnghttp2 installed

While building the bundled curl, -I/usr/local/include appears too early
on the compiler command line causing the compiler to use headers from
an installed libnghttp2 instead of the bundled libnghttp2. Add
USES=localbase so CFLAGS contains -isystem /usr/local/include making
the compiler search the directory after all the -I flags.

PR: 269253