snort3: depend on libtirpc only for musl builds

snort3: depend on libtirpc only for musl builds

The libtirpc package is only needed when building with musl, as glibc
includes the required RPC functionality. This change makes libtirpc a
conditional dependency and adjusts the build flags accordingly.

Building with x86_64-glibc:
...
Feature options:
    DAQ Modules:    Dynamic
    libatomic:      User-specified
    Hyperscan:      ON
    ICONV:          ON
    Libunwind:      OFF
    LZMA:           ON
    RPC DB:         Built-in
    SafeC:          OFF
    TCMalloc:       ON
    JEMalloc:       OFF
    UUID:           ON
    NUMA:           OFF
    LibML:          OFF
...

Building with aarch64_cortex-a76_musl:
...
Feature options:
    DAQ Modules:    Dynamic
    libatomic:      User-specified
    Hyperscan:      ON
    ICONV:          ON
    Libunwind:      OFF
    LZMA:           ON
    RPC DB:         TIRPC
    SafeC:          OFF
    TCMalloc:       ON
    JEMalloc:       OFF
    UUID:           ON
    NUMA:           OFF
    LibML:          OFF
...

Build system: x86/64
Build-tested: x86/64-glibc, bcm27flogic/xiaomi_redmi-router-ax6000-ubootmod (for musl)
Run-tested: x86/64-glibc

Signed-off-by: John Audia <[email protected]>