qingke_rt::interrupt: Fix string literal creation

[agausmann]

Test code:

#[qingke_rt::interrupt]
fn DMA1_CHANNEL2() {}

Before this patch, I get this error message:

error: attribute value must be a literal
   --> life-module/src/main.rs:196:4
    |
196 | fn DMA1_CHANNEL2() {
    |    ^^^^^^^^^^^^^

Output of cargo expand:

#[link_section = ".trap"]
# [export_name = DMA1_CHANNEL2]
#[allow(non_snake_case)]
fn DMA1_CHANNEL2() {}

Output of cargo expand after this patch (note the conversion to a string literal):

#[link_section = ".trap"]
#[export_name = "DMA1_CHANNEL2"]
#[allow(non_snake_case)]
fn DMA1_CHANNEL2() {}

[andelf]

Thanks for the fix.