ext: hide statically-linked symbols to prevent llhttp collision

[stanhu]

libgit2 >= 1.8 bundles llhttp as its default HTTP parser. Without visibility controls, all 119 llhttp_* symbols are exported from rugged.so and collide with the llhttp-ffi gem (used by the http gem) when both are loaded in the same process.

Fix by:

• Passing -DCMAKE_C_VISIBILITY_PRESET=hidden to cmake so libgit2 and its bundled deps (including llhttp) are compiled with -fvisibility=hidden on all platforms.
• Appending -Wl,--exclude-libs,ALL to $LDFLAGS on Linux as a belt-and-suspenders linker-level guard (macOS ld does not support this flag, but the cmake flag covers it there).

[stanhu]

Before

$ objdump -T tmp/x86_64-linux/stage/lib/rugged/rugged.so | grep llhttp | head
000000000010bb30 g    DF .text  0000000000000010  Base        llhttp__internal__c_test_flags
000000000010b8e0 g    DF .text  000000000000014b  Base        llhttp__after_headers_complete
000000000010bc90 g    DF .text  0000000000000009  Base        llhttp__internal__c_load_header_state
000000000010b400 g    DF .text  000000000000004e  Base        llhttp__on_version
000000000010b170 g    DF .text  000000000000001a  Base        llhttp_set_lenient_transfer_encoding
000000000010b7c0 g    DF .text  0000000000000072  Base        llhttp__debug
000000000010b630 g    DF .text  0000000000000023  Base        llhttp__on_chunk_header
000000000010bef0 g    DF .text  000000000000000b  Base        llhttp__internal__c_update_type
000000000010a660 g    DF .text  0000000000000009  Base        llhttp_get_http_major
000000000010aac0 g    DF .text  0000000000000308  Base        llhttp_method_name

After:

$ objdump -T tmp/x86_64-linux/stage/lib/rugged/rugged.so | grep llhttp | head
$

[stanhu]

@carlosmn Could you look at this?