Library is present but LSP complains

Language Server Protocol (LSP)
1

Window_LSP
Window_LSP959×1040 94.9 KB

Why is this happening?

NVIM v0.6.0-dev+445-ga36c6e5df
Build type: RelWithDebInfo
LuaJIT 2.0.5
Compilation: /usr/bin/cc -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions     -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security     -fstack-clash-protection -fcf-protection -Wp,-U_FORTIFY_SOURCE -Wp,-D_FORTIFY_SOURCE=1 -DNVIM_TS_HAS_SET_MATCH_LIMIT -O2 -g -Og -g -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fno-common -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -DMIN_LOG_LEVEL=3 -I/home/inferno/.cache/paru/clone/neovim-git/src/build/config -I/home/inferno/.cache/paru/clone/neovim-git/src/neovim-git/src -I/usr/include -I/home/inferno/.cache/paru/clone/neovim-git/src/build/src/nvim/auto -I/home/inferno/.cache/paru/clone/neovim-git/src/build/include
Compiled by inferno

Features: +acl +iconv +tui
See ":help feature-compile"

   system vimrc file: "$VIM/sysinit.vim"
  fall-back for $VIM: "/usr/share/nvim"

Run :checkhealth for more info

 Language client log: /home/inferno/.cache/nvim/lsp.log
 Detected filetype:   c
 
 1 client(s) attached to this buffer: 
 
 Client: ccls (id: 1, pid: 25132, bufnr: [3, 4, 4, 5, 5])
 	filetypes:       c, cpp, objc, objcpp
 	autostart:       true
 	root directory:  /home/inferno/docs/Projects/dunst
 	cmd:             ccls
2

This is probably isn’t an nvim lsp issue, but rather a ccls config issue for your project. But in any case you probably need to add a flag to tell ccls where that header file is.
You can do that with a .ccls file in the root project directory. See documentation here:

I’d guess that the flag you need might be
-Ipath/to/headers
or maybe
-isystem/Path/to/headers
But it could also be a target name or something else. Just depends on your project.