Add missing import for malloc

[jiahaog]

Internal bug: b/380745221

The use of malloc in L70:

engine/third_party/tonic/filesystem/filesystem/file.cc

Line 70 in ce1eb37

uint8_t* ptr = (uint8_t*)malloc(file_size);

Results in the following error, after an update to the internal toolchain. For some reason, this error only appears when targeting Linux.

tonic/filesystem/filesystem/file.cc:70:28: error: use of undeclared identifier 'malloc'
   70 |   uint8_t* ptr = (uint8_t*)malloc(file_size);
      |                            ^
1 error generated.

This PR fixes it by including the header file.

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